Advice before upgrading

Hello been a visitor to this site for quite some time now back when I got my psp.
Currently my psp is ver. 1.50 phat original battery and 32mb mem stick.

My question is I just received a game that requires 4.05 firmware.
Is there a workaround to prevent upgrading or with my current equipment a way to downgrade later.
I mostly use bookr for reading some of my favorite novels currently reading Steven Eriksons Malazan Book of the Fallen Series just starting book 2 so would hate to not be able to continue reading it.

i recommend going to custom firmware
but i'm in a bit of a rush now so i cant go into detail on how to get to 5.00M33
but that is what i currently use and its the best CFW i've used so far

Ty for the quick response I was pretty sure that was what I needed to do. Honestly haven't used my psp for gaming since lumines came out. I'll see if i can't find it on my own.

update
currently at 3.80 custom firmware
had to get larger mem stick to upgrade it

update 2
up to 5.00m33-6 after figure out first few steps things went alot smoother