If you want to know if you can upgrade during your current rental period, you can find that information here!
Yes, this is certainly possible. This can be done with some services via the customer panel. If not, you can create a ticket containing which package you want to upgrade and which service you want to upgrade.
You then only pay for the days for the period that is running. So you only pay for what you can use. In case of renewal, the full price will be charged.