You really don't need to write a script at all. Click into the field in question, and then go to Records -> Replace Field Contents. You can specify a serial number (middle option), and there's a checkbox at the end of that option to update the serial number in entry options so that the next value is updated in your field definitions. If you have over 7K+ (and counting) records, obviously I'd suggest shutting everyone else out in order to make the changes, because it's going to take a little while, and you wouldn't want anyone creating new records in the meantime.
My god that was easy. Thank you for saving my wall (from my head)!
Use Replace Field Contents with extreme caution on networked files. If another user is editing one of the records you want to modify with this tool, the record is "locked" and it will not be changed. You then get a message telling you how many locked records were skipped and have to try to figure out which ones didn't get modified :smileysad:
I never use this tool unless I am absolutely sure that no other user can lock one the records I'm updating. That sometimes requires running it in a script after hours or taking the field down off the server long enough to run the update.