@Naveen *****
If your extension was issued at OSOS, that is the place you need to go to cancel your work permit and your extension with documents from your employer. And you need to go to Chanegwattana to apply for an extension based on being unable to travel due to Covid situtation.
For those companies eligible to use OSOS, yes, there IS a grace period after the last day. You MIGHT get a grace period when you cancel your extension. IF you COULD get some period, yes, you can go to Chaengwattana before that's up. BUT, that grace period varies from 0day to 21days depending on some conditions (that detail is unknown). And I also saw two cases quitting from the same company got different days (one got 7days, another got 14days). So you need to check with Chumchuri for sure.