Switching from Creator down to Plus (see "How do I upgrade or change my plan?" for the button itself, on your Profile page → Billing) applies immediately, same as switching up to Creator, just in the other direction. If the AI Assistant add-on is active on your account, the switch button won't appear until you turn it off first, see "What is the AI Assistant add-on and how much does it cost?", the add-on is priced per plan and switching plans while it's active would leave it billing at the wrong price.
What happens to what you already paid
The remaining, unused portion of your current Creator period isn't lost, and it isn't refunded back to your card either. Instead, it becomes a credit balance on your account with Stripe, applied automatically to reduce your next invoice (and the one after that too, if the credit is larger than a single invoice covers). Whenever you have one, this balance shows right on your Profile page → Billing, below the plan switch buttons.
Why a credit instead of a refund
A straight cash refund for partial, unused subscription time isn't something we offer outside the specific cases described in "Can I get a refund?", this plan switch follows that same general rule, it just doesn't forfeit the value either, crediting it forward is the middle ground.
Tip: Planning to switch down and your account owns more projects than Plus allows? See "What happens to my projects if I downgrade or cancel?", the project-count consequences are the same whether you get there by switching plans in place or by cancelling and resubscribing later.