30 Hours of Downtime and Zero Leeway: Transformation: S/4 Conversion at Preh

A component that arrives too late can bring assembly lines to a standstill. Preh ensures that this doesn’t happen. Founded in 1919 in Bad Neustadt an der Saale, the company has been part of the Joyson Group—headquartered in Ningbo, China—since 2011. As a Tier 1 supplier, the company delivers control elements and human-machine interface (HMI) systems for vehicles, as well as components for electric mobility, directly to the production lines of international automakers—just in time, just in sequence, worldwide.

Approximately 6,000 employees at locations in Germany, China, Mexico, and other countries keep these processes running and generate annual revenue of approximately 1.5 billion euros (2025). The ERP system is the key driver of the business.

When SAP announced the end of the maintenance cycle for ECC 6.0 Enhancement Package (EHP) 7, Preh had no choice. The company needed to modernize its ERP landscape and accelerate the implementation of S/4. At the same time, the ongoing downturn in the automotive industry was putting pressure on Preh’s financial and personnel resources. As a result, the project had to get underway under challenging conditions—and still be completed by a set deadline!

Right at the start of the project, the company reviewed twelve SAP service providers for this purpose. The IT department and business units evaluated the candidates together; everyone involved had one vote. In the end, three criteria were decisive: proven SAP expertise, specific experience with S/4 conversions, and the chemistry between the teams.

„The first impression we got from the consultants and T-Systems“ presentations was what mattered most to us. Their personal touch was just as convincing as their expertise in problem-solving,” says Holger Koehler, S/4 project manager at Preh. For the automotive supplier, the deciding factor wasn’t the lowest daily rate, but the conviction that the collaboration would hold up over the course of months. The chemistry has to be right! Koehler’s insight: If you have your SAP service provider imposed on you from above, you’ll quickly lose the support of the business units. If, on the other hand, you involve everyone from the start, you’ll gain backing for the difficult phases.

For project management, the team deliberately combined the waterfall and Scrum methodologies. Fixed milestones set the framework, while agile flexibility applies within that framework. Koehler comments: „In S/4 projects, there’s always a point beyond which a purely agile approach jeopardizes the project’s success. If you leave everything open until the very end, you risk missing the go-live date.“

Brownfield, but properly prepared

Following a comprehensive S/4 preliminary study, T-Systems recommended a 1-to-1 system conversion using the brownfield approach. Existing business processes, master data, and integrations were retained. This was because operating two SAP systems in the long term would have jeopardized Preh’s supply chain. For these reasons, a complete new implementation using the greenfield approach was ruled out.

The brownfield approach requires careful preparation. At Preh, two preliminary projects had to be completed before the actual migration could begin: the implementation of the New General Ledger and the migration to the central SAP Business Partner (BP) master data model. The latter consolidates customer, vendor, and contact data into a single object. In S/4, it is the sole entry point for master data maintenance.

Less scope, more control

Both pilot projects were successful; the fine-tuning was intentionally postponed until a later date. At go-live, however, it became apparent that missing details resulted in additional work. Holger Koehler would take a different approach today: „I would pay closer attention to ensuring that the New General Ledger and Business Partner systems are optimally migrated beforehand. It’s better to invest two more months to better map the processes.“

S/4 projects often fail not because of technical issues, but because there are too many projects underway at the same time. Therefore, the team determined which modules should be included in the S/4 migration and which should not. Actual cost calculation via the SAP Material Ledger was implemented in parallel with the conversion because Preh had previously managed this process using Excel spreadsheets.

"I would pay closer attention to ensuring that the New General Ledger
and business partners can be transitioned optimally in advance.
It's better to invest two more months to refine the processes
to represent it more accurately.“

Holger Koehler,
S/4 Project Manager,
Preh

The company, however, put SAP Extended Warehouse Management (EWM) on hold and initially implemented SAP Stock Room Management as a temporary solution. SAP Freight Order continued to run in the existing environment. „There are always conflicting priorities when deciding what to prioritize and what not to. For us, it was crucial to sensibly reduce the scope,“ said Holger Koehler. The issue of interfaces proved particularly challenging. The SAP Business Warehouse (BW) had to be monitored continuously. Interfaces that worked before the migration had to be revalidated after the conversion because APIs operate differently in the S/4 context than in the ECC environment.

One weekend, 102 testers, zero leeway

For more than six months, the team struggled to set a go-live date. They had to coordinate holidays, vacation periods, production cycles, and time differences with the locations in China and Mexico. In the end, only one weekend in November 2025 remained. Preh opted for a global “big bang” approach: all locations at the same time, rather than a phased rollout. A site-by-site migration would have meant that the old and new ERP systems would have had to run in parallel for months.

The planned time window for the go-live was 30 hours. Any longer than that would have seriously jeopardized production operations. „We deliberately chose not to have a Plan B. That’s because it would have made it possible to change our decision and postpone the project,“ says Holger Koehler. „If you have a Plan B, you don’t put all your eggs in Plan A’s basket.“

T-Systems managed the go-live weekend using a global delivery model: Teams in Germany and at T-Systems India collaborated across time zones. Following the migration, T-Systems supported the intensive hypercare phase to ensure the stable operation of the new system and to quickly resolve any issues that arose.

The conversion model followed a five-step structure: sandbox proof of concept (PoC), development environment, quality assurance environment, dress rehearsal, and final production conversion. The dress rehearsal was critical to the project. It validated the actual downtime and revealed final errors that had to be resolved under intense time pressure. On Friday, November 14, 2025, the conversion began. 102 testers worldwide gradually confirmed the release of their system areas. The actual delay was 45 minutes, compared to a planned buffer of one hour. A spot-on execution.

Koehler himself spent the entire weekend in his camper on company grounds. A short walk to the office, a clear head for what mattered. Holger Koehler has been working with SAP for 36 years, the last seven of them at Preh. „Anyone who’s been in charge of SAP projects for so long and sees an S/4 transformation as a life goal doesn’t sleep at home when the time comes,“ he says. When he stepped outside on Sunday evening and took a deep breath, a great weight lifted off his shoulders. A short time later, he had the go-live date tattooed on his body. Not on a whim, but because he had poured his heart and soul into this project.

Faster, leaner, future-proof

The transformation yielded measurable improvements: SAP HANA’s in-memory technology speeds up reporting tenfold. Role-based SAP Fiori applications reduce the number of transaction steps in key user roles by 10 to 15 percent. The brownfield approach was 20 to 30 percent faster than a greenfield implementation, while also posing less risk to ongoing operations.

Preh now has a digital core that supports future expansions through AI, the Internet of Things (IoT), and further automation. The next step has already been determined: the full implementation of SAP EWM.

The project demonstrates that an S/4 HANA transformation under operational pressure can succeed when three factors align: a clearly defined scope, a strong partnership between the client and the service provider, and a willingness to commit to a single plan. „If you leave a back door open, you’re planning with less consistency for the front door,“ says Holger Koehler.