The obvious answer to this question is when your old tyres are worn out. So there are a few factors to consider when determining this, which include, when is it the easiest or the most convenient time to do the change. If we start with the ways to determine if you need to change the summer tyres, it is often due to the fact that the thread depth is below the recommended 4mm, the legal threshold is 1.6mm, but trust me, you do not want to go that low.
If you are planning to anyway change to winter tyres at the end of the season and you are currently close to the depth of 4 mm, then it is probably more convenient to change at the end of the season, if the season is soon coming to an end. If your thread depth is below 4mm, you should change immediately. The thread depth is there to guarantee you your safety. It both ensures you have maximal grip on the road, so that you don’t start skidding when you take turns and it ensures that you can drive safely, even when there is water on the roads.
If you are driving with worn out tyres on wet road, there’s a bigger risk that you lose control over your car as the tyre can’t push the water away to the sides. This is what we call aquaplaning. When aquaplaning occurs you have lost contact with the road, and you can’t control the car anymore, as you are now gliding on the water surface like a boat does. This loss of contact and control over your car can be very dangerous.
Also you need to change your tyres, if they are damaged, if the sidewalls have been damaged, or if there is wear on the structural parts due to hitting some rock or curb. It is recommended to change the whole set, if possible, at least if the other tyres are getting close to the end of their lifecycle. You don’t want to drive with a mixture of new and old tyres.