If you have a flat or any other tyre damage you are of course allowed to change. The rules only say that you cannot refuel at the same time as changing tyres.
This rule might add to it - It takes some skill to drive fast without actually overdriving the tyres.
Nah. I just did loads of slides (managed to avoid the sand traps) but the cra hda an awful setup, so it span when I turned. Best one was when I anticipated rain, so I put wets on, then the weather was sunny. By teh time it started raining, m wets were worn out!