Disney does a very nice job of inter-resort luggage transfers (so long as you are aware of their process)...
First, it is true that there is a rule against taking luggage on the
DVC busses...occasionally you'll see a bus driver who allows it, during an "empty" time of day...but more often you'll see them refuse to allow you to board w/luggage...
Second, you do in fact either take your luggage to Bell Services, or ask Bell Services to come get it (either way works). There is no charge, but tipping is expected.
Bell Services at the resort where you're checking out will deliver your luggage to Bell Services at the resort where you're checking in. All inter-resort luggage transfers take place between 1 and 3 in the afternoon. So there's no chance for getting your luggage back prior to 3 (or so) in the afternoon of the day you transfer.
Your new resort's Bell Services will deliver your luggage to you, but there's a rule that you have to be in the room to receive it (unlike ME, who will just leave it for you if you're not there). Of course, you can stop by Bell Services and pick it up yourself if you want--but it won't be there til some time between 1 and 3 due to the inter-resort luggage transfer "window" they use.
Each time you deal w/a Bell Services person, a tip is expected. But there's no mandatory charge of any kind.
This was the process in place when we changed resorts (or villas within a resort) 3 times on an 8-day trip last year (OKW to SSR to BWV/pv to BWV/sv)...once we figured out the late afternoon arrival and the need to be in the room to get our luggage, it was smooth as silk and a great service...
And yes, this only works among Disney resorts--but it works for all Disney resorts, whether or not they're DVC...it does not work for the Swan, Dolphin, or Downtown Disney resorts...
Good luck to you!