About the IMS to DB2 migration, intuitively I would try to write a translation layer between the two databases. Do you know if this was already attempted or deemed undoable (or silly) from the beginning ?
The programs that use IMS probably do lots of things that are just not done in relational databases for good reason, so if you just translate it you will either get errors in relational constraint or you will configure DB2 to stop enforcing relations, losing most of the advantages DB2 has in the first place.
My last job was at a COBOL shop that used Ordat ti2sql (an emulation layer) to run its HP mainframe code on top of Solaris and DB2. I'm sure similar products exist for other mainframe architectures.
The programs that use IMS probably do lots of things that are just not done in relational databases for good reason, so if you just translate it you will either get errors in relational constraint or you will configure DB2 to stop enforcing relations, losing most of the advantages DB2 has in the first place.