Should I cancel my Non-O extension before leaving Thailand for a Non-B visa application?

Hi guys,

One for the admin.

I am currently on a yearly extension of stay based on non - o ( being over 5O). I'm going back to work for a couple of years - I have a contract in an international school for the new academic year. I've b en asked to leave the country that gets back on a B- visa. I will be leaving for the UK at the beginning of May, where I'll apply for the non B. I currently do not have a re-entry permit and was going to leave on the assumption that my current extension will be voided once I apply for the Non B. I'm registered here in Chiang mai but did my last extension in Pattaya. Is my plan good to go or should I try to cancel the extension before I leave. Bearing in mind I can only get to the CM office if this is the case. Thanks in advance for your advices.