XF 2.1 Changing the price of account upgrades: Effect on current and new purchasers?

I was asked a question about this and didn't know the answer; hopefully someone can share!

Essentially: if someone has an existing group of users that are paying (for instance) $1 a month for a user upgrade via PayPal, what happens if the price is adjusted to $2 within XF? My guess is that existing users would continue to pay $1 in perpetuity, while new users would pay the updated price in perpetuity, but this is mostly a guess as the XenForo 2 Manual doesn't explain what happens in this case (and I suspect it may depend on the payment provider used to an extent).