You can change your monthly plan anytime. There is no lock-in and no penalty for switching.
How to change
Go to Settings > Billing and find the Current plan section. Click Change plan and pick the new tier. Confirm once. The new plan credit lands in your wallet immediately, prorated for the current cycle.
Upgrading
Moving to a higher-tier plan adds the difference in plan credit to your wallet for the remainder of the cycle. For example, if you upgrade from Tester to Serious halfway through the month, the remaining half-cycle's difference is added to your balance.
Downgrading
Moving to a lower-tier plan adjusts your plan credit for the next cycle. The current cycle's credit is not reduced — you keep what was already issued.
What happens to unused credits
Unused credits from the previous plan stay in your wallet. They don't disappear when you switch. Unused credits roll over while subscribed regardless of which plan you are on.
For plan details and pricing, see the pricing reference.