Do I need to cancel my current ED visa extension before applying for a new ED visa for the same school?

I will be applying for my second ED visa next month to continue studies at the same language school. Do I still need to cancel my extension of stay before travelling to Vientiane as I believe you require a letter stating from the school which date your studies will be ceasing on, well as I’m continuing my studies do I still have to do this? I’m guessing that I do because I will be obtaining a new visa and subsequently in the future a new extension of stay after 3 months. Just want to confirm the CORRECT way to do it, cheers.