It's not necessary to downgrade to reformat, just to let you know..
1. Boot ps3 into recovery mode
2. Select option "restore ps3 system"
This will format the hdd and wipe everything (games,saves,etc)
When it is finished, you will be brought to a black screen, asking for your last syscall(last cfw installed)
3. Reinstall your cfw, or a downgrader and then your cfw
if you are on DEX, this downgrader will work for you,
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if you are on CEX, this downgrader will work for you,
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You will probably need to convert back to CEX if you are going to try to install an official firmware, but there should be no reason you would need to install official firmware...
Hope that helps