Can I work part-time in a different field while holding a non-B visa and work permit in Thailand?

Seeking advice on work permit laws...I have a legitimate work permit and non-B visa. I have a potentially lucrative opportunity to work part-time in a different field, coaching figure skating. I remember reading years ago that Thailand had changed their work permit laws, and as long as you had a work permit, you could work at other places than what was stated on your work permit. There was much celebrating in the expat community, which is why I remember it! However, everything I'm finding online says that's not the case, but you CAN get a second endorsement on your work permit by another employer. Is there anyone with experience in these matters who can offer advice? TIA!