I need to change the ETM. Do I need to make class-by-class changes?

No. The data entered in the display Pre-incoming year > Provisional classes > Services depends on the display Pre-incoming year > ETM > Services. To avoid having to modify the services of classes one by one, you just have to make the modification in the display Pre-incoming year > ETM > Services. Also, if you have several ETM for the same level, select all of them before making the changes: you will only modify the common services once.