Will new Logic Board confuse Time Machine?

Hi people,

Had a few probs with my iMac about a month ago and had to get the logic board replaced. Now when I attach the backup hard drive I was using to the comp it mounts as a time machine drive, it believes its the right Time Machine drive in the System Prefs but when it tries to back up it says "Time Machine Error" followed by a spill about not having enough space on the hard drive (as it is attempting to back up my entire internal hard drive (again)). It's as though it's disregarding everything thats already backed up and Im assuming its something to do with the fact the logic board has changed and it thinks it's a different computer altogether? (I could be wrong though)

Is there any way to rectify this or does it mean trashing the previous backups and starting from scratch with new backups (which is something that I could do as I dont think there's anything I desperately need from the backups but would obviously prefer not to do this... just in case 😉

Cheers for any help in advance!