3.71 PSP slim 2003 Downgrade Help

Ok i recently purchased a psp slim its has 3.71 offical firmware and something 2003 not sure what that is i've seen other one's that have 2000 but anyway i also have a Fat psp running 3.40 OE custom firmware is there anyway for me to downgrade/upgrade my slim psp to the M33 firmware 3.71 sorry if i sound like a noob its been ages since i've done any downgrading on a psp ...i can still remeber when everyone got excited over the 2.71 easy downgrader so thats how long its been can anyone help me :D