If you have the installers (one should always download and archive installers for issues like this), I suggest re-installing and then use the correct license key during the re-install.
I have the installer: FM boxed CD, actually. I've tried that several times. Deleted the main FM 12 folder, as well as the ~Library> Application Support > Filemaker folder and the com.filemaker .plist. Restart. Install. The conflicted number persists. Oh, and turned off network access.
Done this several times, btw... this is a consistent issue.
UPDATE: Did a more thorough review of the hardware situation, and it appears I may be able to get the iMac up to OSX 10.10 after all (it may need to have some memory added). If so: I think an upgrade path is in order here.
Still would like to know why I cannot do a re-install, tho.
Solved, sort of. I upgraded the network to v.15 (including upgrading one machine from an old iMac to a newer one). An expensive fix.