How the hell would you accidentally upgrade the firmware if you already has 2.71 emulated on your PSP?! duhh!! It will just say 'no need to update blah blah!
and back with the old days of devhook 0.40 where 2.50 is only emulated, i accidentally accessed the update to 2.60, NOTHING HAPPENED! it just says there's an error. so no need to freak out, and besides updating thingy, im sure there will be lots of notifications/warnings before you could really update your PSP. and you really cant update without your AC adaptor plugged in.