Well now, we use "Pandora Battery" to downgrade our firmware back to 1.5. But you'll need some one with a customfirmware to convert your battery.
Or you can:
If you have 3.50(NOT 3.51 or 3.52), use the 3.50 downgrade guide with a copy of Lumines.
If you have 2.81-3.03, upgrade to 3.03 and downgrade with a copy of unpated GTA:LCS.
For 2.80, use the 2.80 downgrade guide.
For 1.51-2.71 use the 2.71 downgrade guide.
After you're on 1.5, upgrade to M33 directly, you don't need it from a newer sony version.
After you're on M33(or any custom firware), you can revert back to any firmware(both custom and official you pleasure). Chance are bricking (means a bad upgrade or downgrade, you psp can't even boot, only the power light is on) is VERY little if you follow the guide slowly step by step.
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hope it helps.
If you have futher questions, feel free to post in the PSP Help Center.