Not tried it, and dont really intend to. I have heard if you are on version 2 onwards and try to downdrade it can possibly go tits up. I would do a lot of searches, and ask on some PSP forums before attempting.
you can only downgrade if you are version 2.00 or lower, if you are 2.01 or above you are screwed at the mo.
for versions 15.1 or 1.52 you need to upgrade to 2.00 first, then once you are at v2.00 you use the thumbnail hack to downgrade to v1 then use sony's 1.50 update to get it to 1.50 and play copies etc. It is a scary procedure but i watched my mate do it and if you follow atep by step correctly you will have no problems.