I came across a post on Mod the Sims 2 which was about changing the skin of an already existing Sim.

http://www.modthesims.info/showthread.php?t=141881

I followed the directions of the third post in this thread exactly.  After I did what the turorial said I pressed commit on the changes then I saved and I got repeated error message, error while trying to open the package file...  for the neighborhood.   It stopped for a bit but now my Sims 2 game will not load at all.  the bar for loading gets just about to the end then the game crashed and shuts down.  I cannot even go back into Sims PE Neighborhood Browser for the neighborhood i was working in  to fix what I did.  because i get a repeated error message that will not stop popping up.  How can I fix this? my Sim PE version is 0.72.1.33855  I have all EP's and all SP's except celebrations.  Any bit of help you can give me would be welcomed thanks.

It's rather too bad that they didn't mention the vital step of "Backup your neighborhood before doing any editing!" I don't know any way to fix it besides restoring your backup. Do you have any backups lying around, possibly from installing an EP?

It sounds as though you corrupted your sim, and the conflicting data is generating your error reports. Since it isn't too likely that you remember enough of what your sim was like before to manually undo the change and see if it helps, and a missing sim causes a BFVFS, I really hope you have a backup.

(And yes, it is possible to copy just a sim from the backup to the neighborhood, if you know the sim's character number. And since you opened their file you should have the character number.)