PSP with 3.50 or higher FW needs an pandora to get it to custom FW.
I dont see how you got 3.72FW to 1.50FW unless it was already hacked with custom FW. Did you have Official or Custom FW when you had 3.72?
The Slims cant go back to 1.50, mainly because it never had 1.50 to begin with, people tried to run 1.50 on slims, but all they got was bugs and errors, so they used an homebrew called Time machine, which makes the PSP think its 1.50 while it actually isnt.
The newer ones (the ones in stores) cant be hacked as of now. The new 2000 slims (usually the ones with 4.XX FW installed) came with an new Motherboard other than the older 2000 slims, which means it cant read off the pandora battery or the magic memstick.
The 3000 series came with an different motherboard too, its not the same as the newer 2000, so people hoped it was available to downgrade. But no luck.
As of now, people were able to install HEN on the 3000 series due to the SPARTA exploit, which needs the game Gripshift. So I would assume the ability install custom FW, wont be long. But who knows, it might. Guess we have to wait.