All I did was change the header in a hex editor and it worked just fine for me, so here is what I did, in simple steps

1: Open file in hex editor
2: Change the string that read "2.82" to read "2.00"
3: save to backup
4: test on PSP

...

No decrypting, depacking, repacking .. will this method work for 2.71 PSPs? Both work on my 2.80, so I dont see why not. I probably should note this will not work on a 2.5 or lower, possibly 2.6 either .. I just put 2.00 cause I felt like it, you can use whatever value you want. If i change it Id probably leave it to 2.6, for fun.