When upgrading belts, it would be great if the upgrade was applied to the whole belt, i.e. all pieces of the belt connected in straight line including conveyor lifts and ending at inputs/outputs or the belt ends, not just to single piece of the belt. Or at least if the upgrade was able to follow the belt for certain substantial distance such as 100 m to each side instead of upgrading just a single belt piece. If the player doesn't have enough materials in the inventory, the upgrade should be applied as far as the materials suffice.
What is usually the greatest problem are tiny pieces of belt, often built in turns but sometimes built to span minor slope, connecting two long runs of a belt that just don't reach each other or also left behind dismantled on-belt mergers or splitters. They're very easy to miss when mass upgrading and then they are often hard to find on a longer belt or in a particularly complicated bell knot if the player even notices the belt doesn't have the throughput it should have.
There's little point for a single run of belt to use different belt models since the throughput is always set by the slowest part.