Here's the scoop....... I put the sifting part of the litter tray in a garbage bag, dump it, and then with my hand in a second bag (usually the plastic grocery bags we get an abundance of) I rub at the poop pellets (lighly) til they fall off into the garbage bag. If a poop pellet is truly stuck in one of the grids (and they do) I just "PoP" them with my finger...my hand still being in the grocery sack and the pan still being in the garbage bag. I don't find this hard to do at all.

The other secret I have is I dump 1/4-1/2 cup white vinegar in the bottom tray, put the shifting tray back in and gently swish it around....the "swish" and the vinegar removes urine that has dried on the grid. It's like magic.
The vinegar is also a disinfectant and a deodorizes the litter box.

Again with bunnies not liking change, and since they are stubborn, why don't you keep one of the old litter pans next to the new one for the next week then remove the old pan and put the new one in its place. And maybe if they are REALLY stubborn you could put 2 handfuls of Care Fresh on top of the grid for a week or so until they "move" to the new litter boxes completely. It will eventually be worth it! Good Luck!!!