I know it's long over due, Umbra being a mature 1+ year old rabbit, but I have just tonight switched her to timothy based pellets (as apposed to alfalfa, the main ingredient of most pellets). To help ease her into the change, and avoid any upset stomachs, I mixed the old and new pellets for her dinner allotment. Although she at first tried to new pellets without apprehension, Umbra soon decided she didn't much like the change and has only picked at her meal since. I'm hoping hunger will persuade her soon, until then she chose hay over pellets (even though they are essentially the same thing).
About pellets: I switched from Kaytee Supreme pellets to Oxbow Adult Rabbit Blend, because the timothy pellets are less fattening. While alfalfa is fine for young rabbits who are still growing, it is too rich for one who has already matured. Also, it is important when changing the diet of any animal to do so gradually to avoid upsetting their digestion (no one likes poopy pets anyway).