To equip or stow a shield is a standard action (PHB p289)
To drop a shield is a free action
So to change from stance "1H weapon + shield" to "same 1H weapon in two hands" would be either be:-
Standard action to stow the shield on your back
or
Free action to drop the shield in your square
Stowing a shield is a standard action because there are straps securing it to your arm. Dropping it is a free action only after you've taken the standard action to un-equip it.
That sounds like more effort than swinging a sword and swinging a sword is a standard action.I have done live action with plywood shield. The include a handle and an arm band to hold the shield. To drop it you simply let go of the handle and shake it off.