Why is this suddenly an issue? This used to happen way earlier pre G&K when your ranged units went from Crossbows to Riflemen. You're getting two extra eras out of them now, since they can go from Crossbows to Gatlings then to Machine Guns before ending up as Mech Inf.
I'll agree that I haven't liked ending up with melee units that have useless promotions. I feel like this ought to translate somehow when you upgrade it, though perhaps at a small loss. Like, if you have Rank 1 Accuracy and promote to MG, you just lose that perk. But if you have Rank 2 Accuracy, both ranks are removed, but your newly promoted unit gets 1 "pick a new perk" option. If you have Rank 3, they're all removed and you get to pick 2 different perks. Something like that.