If you choose a 2 year regular Tab then yes you can down or up grade at any time to any current in market plan at the time of the change.
If you choose the Tab Large then you are restricted to up/down grading to any current in market Tab L plan at the time of the change.
Also with a Tab, you are never locked in. You can choose to pay off your tab at any time and leave if you want.
Changing plans on self serve is free of charge.
If you call in to ask them to change your plan when you could have done it your self on self serve, then they will charge you for that. But calling in to ask them questions or to request something you could not do on your own on self serve is free always
First of all, I have to say amazing response time! I see Tab Lightweight, Heavywight, and Plus. I I am assuming your "Tab Large" means (Heavyweight and/or but suspect) Plus - neither of which apply to me. Thank you very much for the very helpful response!