it is actualy easyer than which is described i dont know how to do it but i know some suff on it...it happens randomly if u level and quit or something like that at it only gives you the things from your previous prestige meaning if you didnt have the bayonet for the shotty b4 you will not have it now.
P.S. I know you can do it on purpose i just dont know how.
P.S.S Ive seen ppl who did it on purpose in XBL matches