my advice is to just restain them. to do so, go to a beauty supply store, get two bottles of 40% peroxide (its alot stronger than the kind they sell at supermarkets and such) they are about $5 a bottle, then get a tub of a powder bleach called "basic white" for about 16$. IN A WELL vetilated area (this stuff will take your breath away if you are in a small area) dump all the ingredients in a small tub and mix it into a paste,(do not get it on you skin!) then apply the paste to every centimeter of the antler and cover the antler in plastic wrap(suran wrap). then set it in front of a heater for about a half hour. When you unwrap them they will be whiter than snow, then all you need is some burt sianna and burt umber oil paints and some laquer thinner, paint the antlers with sianna, then wipe it off with a laquer soaked rag, the paint will remain stuck in all the cracks and crevices that is what you want. then repeat with the burt umber.
Believe me, there is nothing to it and it will look like new, better than any attempt i can think of to clean off the yellowing. I do taxidermy part time, i re-stain racks several times a year, I can achieve any shade desired on a rack using only those two oil paints i mentioned above.