yes. no question.
i'd be clear with the terms of service so they weren't going to mess me about, but why wouldn't you?
it's not like its an ex wife who cheated on you is it?!!
you run a business, they were customers of yours. they want to come back. so long as they want the service you offer on terms you set, its only good.
there are few better signs of people appreciating you than them trying out another service and then choosing to swallow their pride and ask you back. thats a compliment.