How to quickly select other weapon for a slot in Team Fortress 2?
There are no console commands to change the loadout selection, so there is no way to presets using the .cfg files.
The only way of changing loadout is through the loadout or (as @The Annoying Pyro said) the mini loadout screens.
Loadout saving has been added allowing you to save multiple configurations for different situations. But you must be in the spawn area to swap.