Mixing it up with a pistol and shotgun sounds like it'll be a great improvement for medic.
However, the upgrade mechanic, I dunno that I'm too crazy about that.
If it makes it impossible to carry, say, the medhandgun and medshotgun at the same time due to them being "the same weapon", even though they don't weigh too much, then that's pretty disruptive, compared to if it had simply been four separate weapons.
Maybe to avoid 'sidegrades' later down the line ever being OP, [see FAL, it existing wasn't supposed to make KF1 commando easier, since it was balanced with the SCAR, but the SCAR and FAL both existing made commando easier, since you couldn't carry two SCARs, but you could carry a SCAR and FAL.] TWI should balance it from the get-go so you can carry duplicates of any weapon anyways, [separately, not dual wielding] but maybe that's a subject for an entirely different thread.
I think four unique weapons also would have been more satisfying stylistically as well.
There's a lot to be said for modularity in reality, but for players of a game, the benefits don't shine so much.