If I take out a cellphone contract for 2 years, and I decide to take out a new contract 6 months after taking out the 1st one, then I will be paying for 2 contracts (1st one still has 18 months left).
The same thing goes for month-to-month. You are responsible to cancel a service if you don't need it anymore (usually with a month's notice).
There is no "upgrade" path from 4G to 5G, so if you want 5G, you need to take out a new 5G package and cancel your existing 4G (if you want to cancel it).
You can upgrade or downgrade (switch) between the 3 different 5G packages, and yes, then the provider is responsible to adjust your billing accordingly. (Stop billing you for the old package, and start billing you for the new one - no double billing)
It's really simple and easy to understand.