"Buy subscription" or "upgrade account" button?

The user is on a free trial. When user goes to account settings, there's a button that allows to "buy subscription". When a subscription is bought and user want's to change a plan the same button says "upgrade account".

Is that distinction ok? Or should the button say "upgrade account" in both situations?

But as I think about it user can downgrade account as well to a cheaper subscription, so it woun't be fair to name the button "upgrade account".

Maybe "change subscription"?