Can be a few ways. You can get papers for labor department from current employer saying the are ok to add new employer to your WP, then go to labor dep.with these papers and documents from the new employer and add new position in WP.
Second way which i did recently. It is cancel old work permit and visa same day, and the same day make new Visa and work permit from new employer.
it is more easy than STV actually. These companies just employ you and send you all documents by DHL, so you can apply for NON-B and COE in your country. It takes no more than 3 weeks to prepare everything. But you have to be quarantined for 14 days.
You either have to wait when they will include Canada in STV program or get NON-B single visa for 3 months first and then apply to switch to NON-O in Thailand. You can PM me.