Documentation

To change the usergroup based off the value of a field you'll need to check for the after update or after registration triggers where you want this to be applied. For example the below will work for frontend and backend registration and profile update.

Global
Triggers: onAfterUserRegistration, onAfterNewUser, onAfterUpdateUser, onAfterUserUpdate
Type: Usergroup
User: Automatic
Access: Everybody
Conditions
1: [var1_cb_demo] Not Equal To [var3_cb_demo]
2: [cb_demo] Equal To 1

With the conditions the first one is checking if the field value changed from what it previously was otherwise it would change their usergroup constantly when updating their profile when the checkbox field is checked. As for conditioning against a checkbox field to see if they've checked it you need to condition against a value of 1 or 0|Empty. 1 being checked and 0|Empty being unchecked (it's more reliable to use Equal To 1 or Not Equal To 1).