@Debbie115
You nailed it!
Which is why hosts have to think outside the box, and not only this part but other things here too...
In your case:
You announce the change, maybe have the guest agree in a message to it first. This is handy for clarity of what you're going to do and esp. if it's about you having to increase the amount you'll be getting. And explain to them what you will be doing:
1) Decline their request and follow up accordingly with a Special Offer.
2) Or accept their request as received and follow up with a Change to the booking.
If you need to charge more money, it's safer to choose version 1, because you don't know if guest will actually accept your change after the booking is accepted.