TN-Hen B: Released by Total_Noob

A new revision of Total_Noob's Hen for the firmware 6.20 has been released. Thought I'd put it here for those that didn't know yet. This new revision allows downgrading (even for the unhackable motherboards ie 2000/3000/Go)
Download link for 6.20 TN-B below.

Below is the full changelog, comments in italics are by wololo (developer of Wagic and Half-Byte Loader)

* -Fixed homebrew execution crash on some PSPs (thanks to HacKmaN for debugging)
* -Added 4g model support (this will fix issues for most psp3000 users)
* -Resolved some important NIDs (this means more plugin and kernel homebrew compatibility)
* Added FAKE INDEX.DAT to TN VSH Menu.
* Dump whatever FW with psardumper.
* Copy /F0/vsh/etc/version.txt to /seplugins/version.txt.
* Enable the option and restart VSH, now your PSP version is faked. -> To downgrade just dump a lower FW. (I do not recommend to downgrade if you have a unhackable motherboard until it is proven to work. Downgrading is most likely useless, and on some models will give you a brick)

Downgrading has been confirmed on the 3000 model by many users, I myself haven't downgraded yet as I couldn't care less for lower firmwares, but if you're worried about bricking your psp, then don't downgrade, you have been warned.

Source: wololo.net

Do you know if the Prometheus Loader works with this?

If you mean this one, then yes.

Downgrade?! No way! So that's it then. The unlocked PS1 emulator has happened too and the PSPGo has become the best iteration of the system now :)

People just need to downgrade to 5.03 (was it?) and use the HEN for that version which supports PSX ISOs.

Unless...don't tell me downgrading to 1.50 and upgrading from there to 5.50 GEN is actually possible with this? O_O

I don't think the psp GO can go down that low, I think it'll only downgrade to the lowest firmware that it came with initially, the GO's initial firmware was 5.70 (I think, never owned a GO, so don't know too much about it), while 3000 was around 4.20(?).
Basically downgrading a GO would probably be useless, but then again, I'm not too familiar with how it all works, this is the first time I think that the unhackable motherboard psps (later 2000s,3000s,GOs) has been downgraded. so who knows, it hasn't been confirmed that a GO was downgraded yet.

Well if it's like the PS3 then it can only go as low as the first firmware that very PSP (not model, but the actual PSP itself) had. Maybe it's a mid point like you've suggested (the lowest firmware any of the model has had), but hopefully it's like the Pandora PSPs and can go as low as anything :)

Also a 6.31 to 6.20 downgrader is on the way:
Twitter