Hello,
There is no "switching" between modes.
If you have an account that allows more than 1 user, then all those users can log in at the same time. There is no setting to ONLY allow 1 user at a time.
Ex:
QBO Simple Start: 1 user total
QBO Essentials: Up to 3 different users can be setup and login at the same time
QBO Plus: Up to 5 different users can be setup and login at the same time
