Thanks for all the replies! We do split stays all the time, so moving for one night is no big deal. We've gone to various national parks and have moved daily to see/spend time in various areas to cut down on travel times. We like to stay in the boardwalk area to save travel time to the 2 parks (we walk and avoid transportation whenever possible). Since we've been lucky enough to go to WDW so many times, we mostly know how we will work out doing our priorities which leaves us more flexible with our time. The party night is our last night so I think that will help with the packing issue, we'd leave the necessary things within easy reach and will have a car so can move over easily.
GF is on the bucket list and if we leave YC by check out, we'd have a good part of the afternoon before the party to enjoy the resort. Our flight isn't until early evening so would have time to check out and still go to a park for a bit in the morning. Leaning strongly towards it if it's still available to check the box and cut down on getting back to the resort after the party.