I have read several posts and all have confirmed that a combination of GSM and COBRAS is the perferred method for moving a subscriber betweeen two digitally-networked Unity 7.x servers (Unified with Exchange 2K3).
Concerning subscriber data, what does not come over with the migration. Specifically, does the phone password/voicemail PIN get restored? I've read the docs and I cannot find any reference to this piece of subscriber info.
Is there anything else (that a migrated subscriber would notice) that changes?
Moving Unity 7 to Unity 7 will preserve the PIN (phone password) just fine.
The only PIN issues you have to worry about with COBRAS are when moving from Unity 7 or Connection 1.x (Windows based) to versions of Connection prior to 7.1(3). Other than that PINs are always preserved.
There's a rather extensive section in the COBRAS help about what data is included and what is not off the COBRAS home page:
hopefully that will answer your questions - if not please let me know what's not clear so I can update the doc.