The versatile weapon property says (PHB, p. 147):
Versatile. This weapon can be used with one or two hands. A damage value in parentheses appears with the property–the damage when the weapon is used with two hands to make a melee attack.
A weapon with the versatile property, such as a longsword or a quarterstaff, can be wielded with one or two hands. Does it consume any part of the action economy to switch between using one or two hands on your turn?
Intuitively I'd say no, I imagine it costs absolutely nothing, but I can imagine, at worst, there being arguments for it costing your "free object interaction" (PHB, p. 190). Which is it?
To give a concrete example, if we imagine that a PC's turn starts with a longsword in one hand and a spellcasting focus in the other; they spend their bonus action casting a spell with their focus, and then spend their free object interaction putting that focus away. With only their action left, can they now attack with their longsword using it as a versatile weapon (i.e. dealing 1d10 damage instead of 1d8)?
(Below are some related Q&As, but not specific to versatile weapons, sadly).
The accepted answer to a related Q&A (thanks @NautArch) suggests that:
Taking your hand off the weapon should not require any action expenditure – you are just letting go of it, same as if you dropped it.
You can then use your free object interaction to restore your grip after casting.
The reasoning for the first case makes sense, but the second case isn't backed up by anything, although I can see the logic behind it.
The errata posted in that answer, to me, suggests that it would in fact cost nothing to grip the weapon with a second hand, almost like it's "part of the attack", similar to how the Ammunition weapon property works, but again, this is logic, not RAW.
See also, the "following round" scenario of this question (thanks @Medix2), which involves sheathing a shortsword and attacking with a longbow that requires two hands. This is almost exactly the same scenario as the one I detailed above.
The most upvotes answer suggests:
There is also no indication in the rules that you must use any sort of object interaction to allow the use of your longbow for an attack as long as it is equipped. The two-handed property only specifies that the weapon requires two hands to use, and we have functional use of both. Any interaction with the weapon with both of our hands would fall under the domain of our attack action.
As with the above related question, is there any way to back this up? (I know it's difficult to back up a claim of "there is also no indication in the rules", but still…)
Best Answer
Changing from 2-handed to 1-handed does not require any part of the action economy
Pretty clearly this sort of thing does not require your action, bonus action, or reaction; they simply aren't listed in the list of actions and the like. However, you do also get one free object interaction on your turn, and we get a list of some possible object interactions:
There are a number of object interactions listed here that involve picking things up, drawing things, or putting things away into specific places. Conspicuously missing from this list is anything about simply dropping an item nor anything about changing your grip on an item. This is especially notable because the list includes both opening and closing a door, yet only picking up a weapon, not dropping one. Thus we can conclude that dropping an item and changing your grip on an item do not use up your object interaction, nor any other part of your action economy.
For further justification that this is the intent of the rules we can look at this unofficial ruling (a tweet) from lead game designer Jeremy Crawford:
Letting go of something requires no appreciable effort (no part of your action economy), and there would be no reason to assume that only partially dropping a weapon would require more effort than completely dropping a weapon. Going from 2-handed to 0-handed would not take more effort than going from 2-handed to 1-handed.
Changing from 1-handed to 2-handed is less clear
We don't know very many things about this scenario but we do know that drawing a weapon (going from 0-handed to not 0-handed) uses your object interaction. We also know dropping a weapon and then picking it up would require you object interaction (going from 1-handed to 0-handed to 2-handed). But I wasn't able to find anything on a direct change of handedness.
I was able to find this series of Crawford tweets:
This somewhat supports that going from 1-handed to 2-handed is a non-action, but it's not rules-based, nor is it a direct statement that that truly is meant to be the case.
Personally I would let people go from 1-handed to 2-handed for free
Though I couldn't really find much in-rules support for my stance, it is still how I feel. It seems odd that you could have your action available and a Two-Handed weapon held one hand but be unable to attack with it. Thus far I haven't found this game-breaking and most campaigns I've been in have allowed for it.