Can I change my plan at any time?
In short - yes, you can change your plan anytime you want.
If you were to upgrade your plan to a higher subscription plan, the subscription plan price would be raised, and you will be charged immediately for it.
If you were to downgrade your plan, in that case, you would have to wait for the current payment period and start from the next one. You will be charged less. In that case, you will not be able to process any refund.