When does an ED visa become effective after switching from a TR visa or visa exemption in Thailand?

It's not added to the end of your TR visa since you're changing Visa types. Your ED will start the day you apply and you will lose any tourist visa days you had left

If switching from exempt/TR to Ed visa it will be from the day it’s processed.

Currently the only ED visas you can get from outside Thailand are for university study, and most enter the country right before classes start. It starts when you enter the country. If you don't go to classes it will expire on the first 90 days when you need to extend it. It is a one year visa, but must be extended with the documentation from the university every 90 days.