DB2 database -> does it continue to be platform-specific? (iSeries, LUW, z/OS)

I have found building a relationship table between the old (natural) and new (integer) keys works well in conversion processes such as this. The integer key is built using the standard auto-increment method for the database. Maintain unique keys on the data which forms the natural key in the new tables.

Develop a repeatable data synchronization method that can be applied as often as required. Depending on your needs this can be daily, weekly, monthly, or on-demand. Make the synchronization modular so it can be updated as required. In some development processes you may end up feeding from the new system back to the old system.

As you develop and deploy the system, track which system is the system of record for which data. Move control to the new system as possible. You may want or need processes to identify and deal with changes which occur on the system which is not the system of record.

Manage change overs from the old system to the new. If possible, disable updates in the old system as the system of record is changed to the new system. This will require a feed from the new system to the old.

Use the tools that you are most comfortable with. Your situation seems simple and lasts only for a short time. If you have to keep this up and running for a long time, you might end up by using more advanced tooling, where you define rules for conversions and synchronizations to be performed for you. Doing so is easier to maintain and to document. The learning curve might be a little steep. On the other hand, you could use this case to train yourself. There is no urgent deadline and when it gets more urgent, you probably build the home cooked version within a day.