I have a ticketed itinerary in which one flight (for 2 people) was upgraded using Avios. The Avios haven't been taken yet, but the flight is in April next year.

Based on what I've read here in the past, I have a feeling that the Avios won't be debited for quite a while.

What happens if I use those 40,000 Avios now to book something else?

Mind that I'm not trying to steal anything and I'm not saying I definitely want to do it. However, I will have enough Avios later on to make up for what I'd be borrowing.