If you already have a team with Premium and want to buy Premium for another one, you can easily book an upgrade. Provided that you are a member of both teams with the same account.
Basically, it works the same for all payment methods.
Log in to the team that already has Premium.
Go to "Settings" -> "Premium" and there to "Change Premium Membership".
Here you can change the current number of teams to the desired number.
Note that you can only book up to five teams in the stores.
If you want to book more, check out this post:
How can I book Premium for more than five teams?
In our example, from one team to two teams.
Go ahead and finish the booking process.
What happens now depends a bit on the payment method.
If you book via Paypal/credit card or the Google Playstore, the second team will be added for the remaining term of the first Premium and you will pay a difference for the term of the second team.
So if the first Premium runs until September 2022 and you upgrade to two teams in July 2022. So both end up running until September 2022.
It is similar with the bank transfer, except that you have to transfer the amount yourself.
Pay attention to the purpose of the transfer.
When paying via Apple Appstore, it is handled a bit differently.
Here, both teams will be renewed immediately for the full price.
So if you upgrade in August 2022, both teams will run directly to August 2023, regardless of how long the first team would have originally run.
So you don't pay twice, you'll get a refund from Apple for the remaining term of the first Premium.
Caution. The fact that an upgrade starts immediately only works if the same payment interval is chosen. If you change from "1 Team - 12 Months" to "2 Teams - 6 Months", this new Premium will start AFTER the expiration of the current Premium.
In addition, it is not possible to book the upgrade with a different payment method than the one used for the current Premium. For example, if you booked via bank transfer, you cannot book the second team via Google Play Store.