The downgrader works on a original 6.38 FW (HEN or CFW is not needed to downgrade!) and you can downgrade to Firmware 6.35.
It supports all PSPs (1000, 2000, 3000 and PSP) (newer = 3000). Support for 07G and 09G is added.
I present to you my port of Davee's 6.35/6.31 downgrader to 6.38.
This a very simple app, just put the official 6.20 updater in (ms0/ef0 accordingly)PSP/GAME/UPDATE, and run the downgrader, this will reboot into the 6.20 "update", and allow you to downgrader to 6.20.
Important note: You run the application at your OWN risk. I am not held accountable if you fudge up your PSP.
Another important note: After you downgrade to 6.20, you will get a BSOD (saying that your system information is corrupted), this is OK, just click circle to reset your system information and you will be back to normal.