So, I did the upgrade.
Homebrew - OK
ISOs - OK
Themes - Semi OK. I tried using one of the 3.40OE-A and it does run and the PSP starts but some of the menus don't work and the PSP restart itself. Like with USB and Remote Play, so Themes are off for me. But Remote Play and everything works like a charm.
We'll just have to wait to see how themes work here.
I am uploading the back up for the Flash 0 and 1 in case some start to play with it.
See ya.
Firmware 3.51 M33 Flas0 & Flash1 Back Up =
Link