When I did it i just bought a new battery, im not 100% sure but you might be able to reverse the effect by running the Pandora's battery creator on your psp after you get 3.71m33 on there and there is an option to make a pandora's battery into a normal one. I'm just not sure if that will work with a Hardmoded battery. If I were you I would just get a new battery then you will always have your Pandora's just incase, and besides you cant even install 3.90m33 without a regular battery more then 78% charged.
----------------------------------------------------------------
*EDIT*
Okay so I looked around the fourms a little and found that infact you can get your hardmod battery back to normal
How to convert ur hardmod bettery back 2 normal
For your particular situation what you should do Is hardmod the battery up grade all the way to 3.71m33 with the "magic stick" then proceed to format your memory stick and flow the tutorial above...