Removing the Profile ID from an Animal Crossing: City Folk save for use w/ wiimmfi

I'd like to play Animal Crossing: City Folk on wiimmfi, but would prefer to use my existing save file (rvforest.dat). I'm familiar with hex editing and need to remove the existing Profile ID/Friend Code from my save file (wiimmfi needs to generate a new one, so I'm trying to blank out the old one).

From some research, I've read that the PID is stored in offset 40-43 (assuming this is correct?), and that the checksum needs to be updated:

The data you are trying to edit has a checksum at offset 0x20. The checksum is calculated over 0x111C bytes of data, starting at offset 0x24 and ending at 0x1140.

I modify the data at offsets 40-43 to '00 00 00 00' and then update the checksum at 0x20. However, AC:CF still complains that the save is invalid.

Am I doing something fundamentally wrong or am I missing any information?

I put together a really simple gallery showing what data I'm modifying/selecting, in case it matters: https://imgur.com/a/iSpo3

Thanks for your help!

 

I've got the offsets correct, and the length area being checksummed is correct. I believe there may be another checksum I need to update, but I'm not familiar with the save file format.

Any help from others (who have actually worked with AC:CF save files) is appreciated.