I have a barbarian in one of my groups that likes to switch between his 2h greataxe and 1h battleaxe and shield. To switch weapons after combat begins he isn't gracefully swapping between the two, he drops one set on the ground to pull out the other. It was the only way to realistically handle it in my mind.
Well with a shield by RAW there is no interaction question at all - it takes an action to don/doff a shield, right?