Change the motherboard, and it's no longer the same computer. That's why Time Machine wants to do a new backup. This is normal behavior no different than if you actually bought a new computer.
Before you proceed consider if you may need to erase the backup drive first to enable adequate space. Otherwise, you will have your new backups and the old ones.
This is why one should look at other discussions before posting. Nov 20, 2011, Fortuny answered Markyd's question, and solved my problem with a link to http://pondini.org/TM/B5.html.
I apologize for cluttering the board with redundancy!