The Avios zone of the destination - based on distance. Nothing to do with timezones. Avios redemptions are split into 9 distance based zones, see here:

https://www.flyertalk.com/forum/31910630-post3.html

You can change to another destination in the same zone but moving to a different zone is a cancel and rebook as CT-UK says. You haven't said which airports you are talking about, but be aware just because they may be geographically close, they may not be in the same zone. One might be the extreme of one zone, the other just at the start of the next one up. If the Avios needed is the same in each case (and on the same dates to avoid peak/non-peak issues), then they will be in the same zone.

And since you mention timezones, just to reinforce CT-UKs earlier point that seats are released at 00:00 but that is always GMT, so that is why they say 01:00 since we are currently in BST (GMT+1) in the UK