My situation is a little similar to a post of early 2015 where someone wanted to merge two GW2012 mailboxes for 2 different users into one, and the suggestion was to archive all of one user, and change the fid on the other before importing the archive back into that.
However, my GW system is on a Linux system, and I think I read somewhere that the FID editor does not work with GW2014 and Linux anyway. I am running Open Workgroup Suite in a micro business.
So here's my situation. The older GW file is my own, as is the new one. The old one was the cached copy on my PC before I completely destroyed and rebuilt GW as 2014.2. (There are several reasons for my having taken that route: XEN VMs and a corrupted host VM being some).
I recreated my own mailbox but ofcourse got a different FID. That was after I tried unsuccessfully to use that scripting tool (I forget the name) to create a mailbox with a specific FID.
I can access the old cached copy simply by selecting that file and all my old emails etc are there. Is there any way that I can merge the data in that old file with the data in my new one?