tink_n_pooh already explained the luggage, but I wanted to add something else you'll have to think about = no surprises.
Even though we all love them here, WDW doesn't really do split stays - they consider it two totally separate reservations. So you'll have to rebook your current GF res as two separate stays, with two separate reservation numbers, one at GF and one at YC.
From your ticker it looks like your vacation is in 5 days? Is your current res a package or room only? If it's a package you need to make sure they waive any change fees since you're so close.
Also does your current GF res have
DDP? If so, and you will want DDP again for your new 3 night YC res, remember that the new YC res will also have to be a package. That means your YC res will need at least one day tickets for everyone all over again if you want to add DDP. (Yes, you can buy the full length of stay tickets with the first res, but you'll still need the one day tickets to get DDP on your new YC res. You can always save them for your next visit).