Welcome to FT! You can call JAL and ask if they have a baggage handling agreement with UAL. If they do ask about any restrictions and make sure you let the person checking in your bags at the airport know the bags final destination. They will want to see the UA flight info. I would suggest anything you need for the transit hotel you keep in your carry on. Depending on the number of bags you have you may be able to check one all the way through and pick up one at Changi and recheck it in the morning. It would be best to ask at the airport unless anyone else here can comment.