Brewology

Yes, now you would need to upgrade to a custom firmware. I would suggest either 3.52M33-4, or 3.90M33-3. I like 3.52M33-4 because it comes with the 1.50 addon (that's used only on phats, and lets you run homebrew apps that were originally made for 1.50), and it supports all the old apps I used to use (I.E. any apps that use wifi, like PSPWeather). But on the other hand 3.90M33-3 is newer, more support for using no UMD in the UMD drive while you play games off your memory stick, however; you can't run a lot of apps that use wifi, and the autoboot feature is broken. The autoboot feautre was often used to automatically run programs, or like a custom power animation. There called cintro's btw (Custom Intro). So you could first try out 3.52M33-4 (First install 3.52M33 then add the 4th patch) and then try 3.90M33-3 (Same as 3.52, install firmware then 3rd patch). But you do have to be 3.52M33-4 before you even install 3.90M33 anyways so yea.

Hope that answered your question :p If you have any other questions ask. I tried to cover most stuff you would've asked anyways. Also, once you upgrade to a custom firmware, and say you want to go to 3.52 from 3.90, it won't be a problem. You don't have to "downgrade", just install it like normally.