You can not change the reason of an issued and used visa.
You can change the reason of the Extension of Stay.
You have to discuss with your new employer if they can meet the requirements and with the BOI how to cancel your current Extension of Stay (if you already have one) into the new Extension of Stay based on your new employer.
If you can not travel, you have to apply before your last day of the current Admitted until or permitted up to date mentioned in your passport.
IF you are in quarantine, the medical facility offers help and a letter to bring to the Immigration office to apply for an Extension.
If you do nothing and just go into overstay, the penalty is 500 thb a day with a maximum of 20.000 Thb, depending on length of overstay black listed to re-enter Thailand.
If you get caught with overstay, another can of worms will open up.
Prevent to go into overstay, it is not that difficult.
Problem is that most Embassy do not provide a letter that people are unable to travel. Just visit your local Immigration Office but have a plan B ready.