Your PSP is now on modern CFW, but your old data needs attention.
The Ultimate Guide to Upgrading Your PSP from 3.52 M33 to 6.60 Custom Firmware
Upgrading a PlayStation Portable (PSP) directly from an ancient Custom Firmware (CFW) like 3.52 M33 to a modern version like 6.60 requires careful stepping. You cannot jump directly to 6.60 because the old M33 updater lacks the modern keys to read newer official update files. Attempting a direct leap can result in a soft-bricked console. psp 352 m33 upgrade to 660
Built upon the original 3.52 Sony OFW. It introduced the 150.PBP kernel add-on for 1.50 homebrew compatibility but lacked modern features like CID encryption bypass for PSN or ISO cache.
The Ultimate Guide: Upgrading Your PSP from 3.52 M33 to 6.60 Your PSP is now on modern CFW, but
Your hardware is now completely modernized. You can take advantage of several modern conveniences:
Because 3.52 M33 uses an older update verification system, standard 6.60 firmware installers cannot read its structure. You must bridge the gap using custom firmware 5.00 M33-6 as a stepping stone. Attempting a direct leap can result in a
Better support for running PlayStation 1 classics.