If you're considering a downgrade, the most important thing to know is when it takes effect — and it's sooner than you might expect.
Clicking Change Plan to move to a lower tier changes your account's plan the moment you confirm it — the same request that updates your billing also updates what your account is entitled to. Nothing about a downgrade is deferred to a future billing date. If you're relying on a lower tier not taking effect until later, plan around it taking effect right away instead.
Downgrading never deletes data you created while on a higher plan. Everything you'd lose access to is kept exactly as it was, and comes back automatically the moment you upgrade again — you don't have to recreate anything.
If a downgrade takes your account below Business, your team members' accounts aren't deleted or disconnected from yours — they simply can't log in anymore while your plan is below Business. The moment you're back on Business or above, they can log in again with no need to be re-invited.
This is worth being precise about, because the product describes this moment two different ways in different places, and neither one quite matches what actually happens: it's not that the seat is "suspended," and it's not that access is "revoked" in the sense of being torn down. It's a check at login time — if your plan doesn't currently include team seats, that login is refused. Nothing about the team member's account itself changes.
Upgrading uses the exact same mechanism as downgrading — the same request that updates your billing also updates what your account is entitled to, immediately, not at your next billing date. Everything listed above comes back the moment you're back on the plan that includes it, automatically, with nothing to recreate: groups, category profiles, threshold alert rules, and team member logins.
Your device limit works the same way too. If you were already at your old plan's device limit, the new, higher limit is available immediately — the very next time a blocked device tries to register, it succeeds, with no separate step to unlock it.