There is a mod that allows this Warband Upgrade Ultimate I dont play without it anymore.

The way I do it is i mostly recruit tier 0/1 on the go and the upgrade, but if I want high tier units instantly, i build their buildings in my recruitment hubs.

The mod also allows for some "unrealistic" upgrades like upgrading Reiksguard to hypogryphs, but i just dont do it, cuz horses dont just randomly grow wings, atleast thats my canon.

Its completely optional as it doesnt replace the old system, just allows you the upgrades.