Apparently, you cannot do any changes to your work permit (you have to obtain a new work permit if you want to do changes) and cannot work under any category of work that is not prohibited (as long as you have a work permit) in Bangkok, as I was said by the Labour dept.(This is a new law as they said)
TLDR : Answer Summary
A new law in Thailand states that one cannot make changes to their existing work permit; a new work permit must be obtained for any alterations. As long as individuals hold a proper work permit, they are permitted to work in any job that is not explicitly prohibited by law.