Feeding trials have shown that reducing pellets won’t hurt a growing rabbit
(from HRS http://www.rabbit.org/journal/3-4/pellets.html)
In feeding trials in which pelleted feed was reduced to 50% of normal intake and the diet was supplemented with greens, young rabbits maintained normal growth. When the amount of pellets was reduced to below 50% of “normal,” growth rate declined (Pote et al 1980). These studies indicate that even young, unaltered rabbits do well on a reduced pellet diet. Since most of our house rabbits need to lose weight rather than gain, reducing pellets below 50% should not affect spayed or neutered adults adversely
Remember that pellets were invented for the commercial rabbit industry. You want to get your rabbit used to eating hay as it keeps his teeth busy. Rabbits need to be chewing most of the time to keep good dental health. I am always keen on getting kits hooked on hay early for that reason.