Contracts don't have to be scary worrisome things, seriously. Consider a marriage contract, for instance. A contract is simply a statement of how each individual is going to be considered in the future when making decisions of any kind.
Contract writing/signing time is your time to say "Actually I'd like to own the IP myself, and let you sell the game as part of your KS but not beyond that" and then if the other party disagrees you have that conversation. You know, BEFORE everybody has done a bunch of work on the thing, or even as is the case here, the work is done and has already been sold to people.
There's nothing at all wrong with a contract, and if you ever present someone with the idea of drafting a contract together and they get antsy about it, that is your first RED FLAG! Seriously the stories of people being fucked in one way or another because of no signed contract, where they have no recourse at all, are plentiful. Do yourself AND your partner(s) a favor and always do a contract.