Update about the new rules on WP. Looks like it is confirmed. At least in Phuket. No guarantee that all other offices follow suit right away. Even though they should..
"The Chief of the Phuket Provincial Employment Office, Kattiya Pandech, has confirmed to The Phuket News this week that new provisions issued by Royal Decree in March are in effect, allowing foreigners with work permits to legally work in any field, anywhere in the country, for any employer as long as the work being done in not on the list of occupations prohibited to foreigners.
This means that foreigners who have been issued work permits are no longer restricted to performing only the type of work listed in the work permit, or at the exact location listed in the work permit, or for the employer listed in the work permit, or even in accordance with the conditions listed in the work permit, Mr Kattiya told The Phuket News.
There is not even any legal requirement for foreigners to inform his office of any changes, he added."
TLDR : Answer Summary
Kattiya Pandech, the Chief of the Phuket Provincial Employment Office, has announced that new rules from a March Royal Decree are now confirmed in Phuket, allowing foreigners with work permits to work in any field and location across Thailand as long as their work is not in a prohibited occupation. This marks a significant change as foreigners are no longer restricted to the job or employer stated on their work permit, and there is no need to report any changes to the office.