Try boiling a #10 can 1/2 full with water and submerging them, I 've done this and it gets about 90% of the old lube off, just wipe the bullet with a towel while its warm to get the rest off.
this is the recipe for bullet lubricant I use for both muzzleloading and BPCR. I pan lube all y cast bullets. Mix and microwave until liquid and you can pour it into molds for future use. feel free to modify it a bit to get the consistancy you are looking for.
Like they said.. boil them in an old soup can. Hold the nose of them with a pliers and dunk them and nylon (no metal) brush them. Normally that old lube falls off in chunks. You're not going to get that bullet 100% clean but good enough to re-lube.