Honestly, I'd lean towards making it more restrictive, not less. "It is an action to change what you are holding in your hands." Keeping a hand free than becomes an actual tactical benefit, rather than worrying about situations where you're constantly juggling to make sure you have both hands doing something.
Basically, if you want to be casting spells in combat, don't use a two-handed weapon or dual-wield.
What I like about your suggestion here is that in most of my time playing, the Versatile property is completely useless. Either one-handed weapon users have another weapon or a shield in the other hand (thus making the possibility of wielding the weapon with two hands moot), or if they intend on wielding a weapon in two hands, they just use an actual two handed weapon for the greater damage dice.
But with what you are stating here... Versatile actually comes into use. A EK cannot wield a greatsword, then let go with one of his hands to cast spells with it, because the sword is too heavy to hold up that way (while keeping it up in position to attack or defend with next turn). Thus, an EK must use a longsword instead because it's smaller enough that he can. *But* because of the Versatile property... on those rounds when he doesn't cast a spell, at least he can put his free hand onto the weapon and use two hands to wield it to get that extra point of damage.
That makes much more sense to me and makes use of a weapon property that I've never really experienced a need or use for. Good show!