Plan changes happen in Settings → Billing. The behavior depends on direction:
Upgrades
Immediate. We charge the prorated difference between your old plan and the new one for the remainder of the current billing period. The next invoice is at the new full rate.
Downgrades
Take effect at the start of the next billing period — you keep the higher tier’s features for the rest of the current cycle. No partial refund; you’ve already paid for the period.
TIPIf you’re going from monthly to yearly, the upgrade is treated as a fresh annual subscription. The remainder of the current month is credited to your account.