I have downgraded mine and re-upgraded back to 2.0 twice now with no ill affect. BUT you can make an expensive brick if you use the wrong downgrader. Some people are packaging a corrupt flash just to kill youPSP and are saying its the downgrader so make sure you get from a reliable source if you are going to try it.
With this being my first post on here i can understand that my post might well be viewed with a certain amount of scepticism.
ive been modding psp's since they first showed up on the scene earlier this year and the best one that ive found to upgrade and down grade are the Japanese units (there are the ones that are the 100X series).
downgraded mine and a mates too no probs. back to 2.0 now tho for liberty city stories! so works alright but just make sure you get the files needed from a reliable source! do that and follow the instructions properly then you'll be fine.
I've downgraded my machine and it worked and I reupgraded again and it was fine.
I would watch out what programs you use etc as there are a lot of sites claiming to have all the stuff but infact it stops your machine from upgrading so you can't play new games which require newer firmware version's and it's been labled "bricking" and I'm sure some programs just **** it up fullstop.