Answer
The sequencing decision usually comes down to blast radius. A finance close process, a customer-facing order entry screen, and a shipping confirmation routine carry very different risk profiles if something breaks mid-cutover, so the safest first phase is usually the one where a rollback plan is realistic within a single business day. Buyers should ask the implementation partner to name the specific tables, RPG or COBOL programs, and interfaces that phase one touches, and to confirm those objects can run in parallel with the legacy versions during a transition window without corrupting shared Db2 for i data.
Cutover mechanics matter as much as module selection. Ask how the team handles journaling during the switch, whether a fallback to the prior process is a same-day option or a multi-day restore, and who owns the decision to abort if reconciliation numbers do not match. Teams that skip a defined go/no-go checkpoint tend to discover data integrity problems weeks later, after both systems have already diverged. A good phase one also gives end users something they can point to as a win, since that credibility is what makes phase two easier to schedule and staff.