5e rules do not require an Action to change weapon, BUT you need an Action to remove a shield and you of course cannot change armor.
I see no reason to make BG3 more restrictive than the core rules.
You get 1 free action a turn. It costs an action to draw or sheathe a weapon. Given the fact that players, if making the choice that they need a new weapon, would drop their old one on the ground and use a free action to unsheathe the new weapon I usually just say its a free action to change out a weapon, but it is once a turn. It is the effect of the economy on that and enemies arent going to go scrabbling for a shortsword so lets make all of our lives easier.
You are right, to equip or unequip a shield is a full action.
What they havent addressed is clerics who shield and mace it. You cannot, without the War Caster feat, cast a spell if both hands are occupied. Holding a weapon and a shield means you cant cast. So you sheathe your weapon to cast. You cannot unsheathe it now as you used your 1 free action. If someone left your threat range and you were to make an AOO it would be with your fist. That is ultimately the effect of that economy. War Caster is nice for clerics that fight - the ones in the back mainly benefit from the concentration advantage.
If a sword and board wanted to go to 2h glaive id say that is going to be their action and free action, they have bonus and movement left. Otherwise, its never been an issue with the 5e mechanics to do mid battle weapon adjustment.
