Each plan includes a fixed number of teams: 4 on Club and 12 on Club Plus (see Club vs Club Plus). Trying to create a team beyond that limit is blocked outright, with a message pointing you to contact support rather than an instant self-serve upgrade. Upgrading itself is a separate step through your billing settings.
If you downgrade to a smaller plan and it leaves you over the new limit, none of your existing teams are touched, archived, or removed. You just won't be able to add further teams until you're back under the limit or you upgrade again.