I've just seen the announcement for this and the requirement to uninstall FileMaker Server before installing. Assuming this isn't a late April Fools article (if so, you got me!), if you run a Windows server do yourself a huge favour and ensure you create a compressed copy of your inetpub folder and backup your current IIS files before uinstalling anything.
In our experience uninstalling FMS on Windows can destroy your web sites published on the server and, without these backups, will leave you digging through your backup tapes/disks.
There are some good articles on the web on how to do a safe uninstall/reinstall, which does include the use of appcms.exe (IIS backup/restore) and switching ASP.NET 2.0 between 32-bit and 64-bit versions of Windows, which we've always had to force on our 64-bit servers.
We could put together a quick summary of procedures if it would be of interest anyone.
I've no experience of uninstalling FMS 12 on the Mac OS, but hope the above helps prevent someone going through what we had to learn the hard way!