About an hour ago, i released a psp downgrader that i suspected could downgrade anything. However, this was banned because it was untested and might brick your psp. So instead I'm going to tell you how I made it and see what people think.
1. Download the PBP Unpacker and FW 1.50 for psp.
2. Open the FW with the PBP Unpacker and extract the files.
3. Click 'New' and put all the files (PARAM.SFO, DATA.PSP, DATA.PSAR and the ICON0 if you want a picture).
4. Right click on the ... box next to the PARAM.SFO and click on SFO Editor.
5. Double click on the box UPDATER_VER and change '1.50' to '3.41' (without quotes).
6. Click File then Save to PBP.
7. Save the file EBOOT.PBP in your memory stick under PSP\GAME\UPDATES.
8. Load the file on your psp.
NOTE: You could probably change the '1.50' in step 5 to any nmber higher than your version of psp as long as it has 2 numbers after the decimal point.
I believe that this works because changing the '1.50' to '3.41' makes the psp think that the file is simply an upgrade to version 3.41. If anyone can prove me wrong and say why this wouldn't work is more than welcome to post their views.