FileMaker 12 introduced a new file format: .fmp12. Because of this, FileMaker 12 will only be able to host, view, and modify FileMaker 12 files.
FileMaker 11 uses the .fp7 file format which makes it compatible with the FileMaker 7-11 versions.
Converting databases to FileMaker 12 is as easy as opening the previous version files in FileMaker Pro 12.
Here is a Knowledge Base article that might help you out with FileMaker 12 conversion:
Hope this helps,
I would like to add that FileMaker 12 no longer supports Mac based PowerPC workstations, version 11 was the last to do so, but version 11 can't access the new .fmp12 formated files. This means that if there are PowerPC Macs still using Filemaker, they will not be able to use the Filemaker 12 client, which, in turn, means the clients will not will not be able to connect to the newly converted files on the Filemaker 12 Server. In our companies case, we own FM 12, but will continue to use FM 11 untill we spend quite a bit of dollars to upgrade specific users' workstations to Intel, even though theses older PowerPC based machines work perfectly fine doing what they do. I would like to note that this is not a complaint, it's just the facts that our business will have to deal with and something to consider before upgrading.