In 4th Edition D&D, Swordmages have a feature called Swordmage Warding, which reads:
While you are conscious and wielding either a light blade or a heavy blade, you maintain a field of magical force around you.
This field provides a +1 bonus to AC, or a +3 bonus if you are wielding a blade in one hand and have your other hand free (not carrying a shield, an off-hand weapon, a two-handed weapon, or anything else).
So, if I have a longsword in my main hand and nothing in my off-hand, I get the +3.
However, longswords are Versatile, which reads:
Versatile weapons are one-handed, but you can use them two-handed. If you do, you deal an extra 1 point of damage when you roll damage for the weapon.
I can't find anything in the Rules Compendium or Player's Handbook about what sort of action switching from a one-handed grip to a two-handed grip is, but I'm assuming it's a free action.
In that case, can I put my off-hand on my sword before attacking on my turn, get the +1 to damage, free my off-hand after the attack, end my turn, and then continue to get +3 to AC on everybody else's turn?
If so, what about when I use my Aegis of Assault Immediate Reaction, which is described as follows:
If your marked target makes an attack that doesn't include you as a target, it takes a -2 penalty to attack rolls. If that attack hits and the marked target is within 10 squares of you, you can use an Immediate reaction to teleport to a square adjacent to the target and make a melee basic attack against it. If no unoccupied space exists adjacent to the target, you can't use this immediate reaction.
Could I use a free action to put my off-hand on the sword before the melee basic, and remove it afterward? Note that the Rules Compendium says that:
A creature can take free actions on its own or anyone else's turn.
This would seem to make using a Greatsword as a Swordmage rather pointless.