I'm not quite sure what input on_player_alt_reverse_selected_area corresponds to, but it seems like you could fit an extra selection mode in there.

This new mode would upgrade every belt, underground and splitter that the mod knows about to the currently selected tier.

So if you're playing base Factorio and you have "Fast" tier equipped, it would order this upgrade:

• Transport belt -> Fast transport belt
• Express transport belt -> Fast transport belt
• Underground belt -> Fast underground belt
• Express underground belt -> Fast underground belt
• Splitter -> Fast splitter
• Express splitter -> Fast splitter

Alt reverse would be shift right click - all though why not just use the upgrade planner?

Just convenience, and you already have a mode which acts as a filtered Deconstruction planner.

In the base Factorio setup, I would need to reserve three Quickbar slots for upgrade planners to easily change to each type of belt, or spend time going into the Blueprint Library. When using mods that add extra belt tiers, the work of setting up all of these planners increases quadratically.