@Louis ******
you'll have to check with the relevant agencies if they will let you cancel without paperwork from your job.
Normally you cancel your work permit first with the letter from your employer, then bring the work permit cancelation receipt to your local immigration office to cancel your extension.
This would need to be done on the same day usually. But you'd also need everything in place already to apply for a new work permit and extension the same day.
It's usually impossible without cooperation between both jobs and immigration.
You'll likely need to cancel everything and leave Thailand to apply for a new non-B in a neighboring country, assuming you're able to cancel everything.