i've downgraded from 2.5 - 1.0 successfuly
.....and i have no idea how - i have an original jap version 1 psp, i upgraded all the firmwares along the way so i could play all the latest games, most recently GTA LCS.
so, while awaiting homebrew to become available again, i left my psp to gather dust (thought gta was disapointing) so today i boot it up - no luck. dead. oh its the battery ok, so i plug it in and boot up burnout legends. then it decides to crash with a black screen with all sorts of gibberish on it - not hex codes or anythin just crap (letters and symbols in tripples ie. 333 ;'# bbb hhh mmm nnn jjj etc..) half the screen was full of this so i hold the power switch for 10 seconds to kill the system
when i reboot, im asked for all my details again then to boot burnout i need to upgrade to 1.52! :-0 so i decline and recheck the system info and it reads as firmware version 1.00 - WEIRD!
has this happened to anyone else?? anyway sweet for me, im going back to version 1.5 :D