When I was new to traveling, I ended up with at least three different Marriott Rewards numbers. At the end of last year, I received a card in the mail letting me know that one of the accounts was going to expire and that I would lose those points. I just called in and they were happy to transfer those points to the only account I remember having. In fact, she even found the third account and moved those points, too.

Not sure how that 2nd account had my current mailing address.

So just make a phone call. Unless you are doing something against the T&Cs, you should be fine.