Firmware/Hacking Question

Hey there everyone. I was very active on here, I was even a moderator for a short time, until a couple years ago. I no longer have a PSP, I haven't even touched one in probably more than a year, but a close friend of mine does. (older slim version- a.k.a. PSP 2001) I borrowed it for a few days and through my PS3 I downloaded a few a demos.
This is where my question comes in and I think it will be an easy one.
PSP has 4.01 M33-2 installed on it. (ancient I know) Some of the demos I just downloaded say to update to version 5.51 or higher for the demo to play.
If I do this update, will the ISO games still be able to play?
Does this update the custom firmware or reset the system to a SONY firmware version 5.51 or higher?

If you want to keep hacking and play those demos at the same time, update to 5.50 GEN-D3, which supports 5.50 and up(6.30...). If you want the firmware, contact me. Also there are others that support those games, put you need to do alot of steps if you want to stay on M33.

Thanks for the reply.
I don't really care, and neither does my friend, if he stays with M33. We really want a firmware that will still let us play ISO's AND the demos if possible.
I still have a pandora battery, if that would be needed, but it has been so friggin long since I've done anything with a PSP. I just tried to play FTB2 and got totally smoked! LOL
If you could send me the firmware that would be appreciated, but installing it correctly would mean I would need some steps explained to me. Everything is slowly coming back to me so it's not like I'm gonna ask a bunch of noob questions but it has been a while.
Thanks again.