Can I schedule a future price change without affecting bookings already made?

Two separate questions hide inside this one, and they have different answers.

Can I queue up a change for a future date?

No. Price Groups (Inventory → Price Management → Price groups) are a live grid — whatever you type and save takes effect immediately, for every booking made from that moment on. There's no "effective from" date, no way to schedule a change in advance. If new prices need to start on a specific date — 1 January, say — someone has to be the one to open the price group and make the edit on that day.

Will changing prices now affect reservations that already exist?

No, and this is safe to rely on. A reservation's price is calculated once and stored on the reservation itself — when it's created, or whenever something specifically re-prices it (adding or removing items, changing dates, applying a discount). Editing a Price Group afterwards never reaches back into existing reservations and recalculates them. This holds no matter how far in the future the booking's rental dates are — a reservation made today for a rental next August keeps whatever price it was quoted today, even if you change the price group in between.

So in practice: whenever you make the change, it only affects bookings made after you save it. Every reservation already on the books — regardless of when it's due to travel — keeps the price the customer originally agreed to.