When is the effective cancellation date for my ED visa after visiting immigration before switching to a Non-B visa?

A user is planning to change their education (ED) visa to a non-B visa by first obtaining a cancellation letter from their university and going to immigration for cancellation in early December. They are unsure if the cancellation becomes effective on the date they visit immigration or on the date stated in the letter (December 20). A commenter warns that the Thai Embassy in Vientiane will switch to an E-visa online system on December 20, which may affect visa applications processed there.