I have personally performed two database merges like you're describing before. Both involved SOP, RM, and GL. One also included PM and POP.
The difficult thing about this is that there is no real "tool" available to do this. This I think is primarily because there can be so many inconsistencies in the two databases. For example, if the same customer is in both databases, do they use the same ID? If you answer yes, then you are better off than most.
You certainly will need to combine GL info, so you are doing well in mapping that out.
Regarding other modules, try to take as minimilistic an approach as possible. The more you choose to combine, the longer you'll be combining. Remember that you can always have the old company available for inquiry purposes only, so you hopefully will be able to resist the request to merge in historical data, which I did for one of the two merges. It's almost impossible to merge historical data perfectly. Since the transactions were created in a different database, it's a lot like trying to stick a square peg in a round hole.
That's all the advice I have. Keep it as simple as possible, and good luck.
Anyone else have ideas?
Ron Draganowski
Senior Solution Developer
Olsen Thielen Technologies, Inc.
rdrag@ottechnologies.com
www.ottechnologies.com
Find me on LinkedIn: http://www.linkedin.com/in/rondraganowski