Never upgrade the machine's OS if your main concern is losing function of old software. Always check to see if the old software you want to continue using, will function with updated OS. Never expect that old software will work with new OS.
There are no issues with Yosemite and FTM Mac 3. Works great!
You may take this opportunity and upgrade to the newest version FTM Mac 3. It's light years ahead and infinitely more stable than the creaky first version for Mac.