Well, I have never personally put in a motherboard and left the existing drive and OS untouched. From what I understand though, this can sometimes give you problems if you leave it. It is worth a shot though I would imagine, you have nothing to lose...its either going to work, or you will have to redo the OS...won't know until you try.
I'd wait until someone more knowledgable replied though, cause I dont trust my judgement yet when it comes to computers (rightfully so...lol)
Retirement isn't all it's cracked up to be