After you have submitted QSL cards for card checking, and have received Credit Granted status from ARRL, you are correct, there is no niffy way to change all those QSL's Credit Granted status in Log4OM, except manually, one by one, as Chuck has pointed out. But, you can download the DXCC QSL Download - ADIF file from the LotW website, then use my Credit Granted import program to automatically update all your Credit Granted QSL statuses. I believe you have used my import program before.
Let me know if you have question, or need another copy of the program.
Doug - W7DRM
Your ideas for making Log4OM even better.