Hello, can i convert my visa on arrival to a non immigrant work visa if i get a job once i enter Thailand?

Hi, I have a non-imm work visa which I have to reapply for extension of stay every 3month and then reapply for a new visa abroad after 365 days. I am resigning from my job and going back to Europe in April. My current extension is until 7 April but my flight is 27 April. Should I ask my employer to give me a resignation letter dated end of April, then extend once more at Chaeng Wattana at the end of March, get 1 month extension and then cancel the WP and visa just before I leave Bangkok? Do I have to cancel my visa even though I don't have a 1year extension?