there should be no need to cancel a DTV, what you should do is speak to your immigration office about CONVERTING the DTV to a retirement visa inside Thailand. According to the announcement of the DTV in the Royal Gazette this is the way to do it
I believe you should be able to convert a DTV inside Thailand with appropriate documentation. Note that is to convert from one to the other, not to cancel one and then apply for the other. The original announcement of the DTV in the Royal Gazette stated this could be done but I've yet come across anyone who has said they have done it. You can but try.
Will the company you want to work with issue the paperwork to apply for the Non-B inside Thailand? If so you can in theory apply to convert your DTV to a Non-B at an immigration office inside Thailand. Note convert and not cancel the DTV, although I'm not aware of anyone having gone through the process it stated in the Royal Gazette entry announcing the DTV that this was available