Brewology

im not too familiar with it, but there is a 2.71 firmware downgrade which requires a TA-082 motherboard here:
http://www.pspbrew.com/forum/viewtopic.php?t=6810

there is a 3.09 or lower downgrade using an unpatched version of GTA:LCS here:
http://www.pspbrew.com/forum/viewtopic.php?t=7034

or there is a 3.50 downgrade using an unpatched version of Lumines I, you might have it since it came out a while back.
http://www.pspbrew.com/forum/viewtopic.php?t=8427

If I recall theres an Auto downgrader for 2.5/2.6 that requires GTA:LCS (unpatched) a bit of digging round the net might help you out. It's the one I used when I was on 2.6, and it worked fine...in fact I used it twice