How would I switch weapon hand attachment dynamically?

For example, I have a character with a two-handed sword-spear (like this) and when that character is walking he is using it as support with his left hand in order to walk faster, but once he starts running and/or attacking, he picks it up and puts his right hand near the base of the weapon. Here is where I am in my animation, and in unreal engine 4 in my animation tree.

Then when he then swings it at enemies, his left hand slides along the shaft of the weapon and meets his hand at the bottom in order to get the weight behind the swing like when you split wood with an axe (example - not the same direction of the swing, but the position of the left hand is the focus).

My problem lies in the fact that my character has his attachment points on his left hand in order to get the weapon to follow his hand when he walks with it, but the weapon will follow the left hand even I try to make the hand to ‘slide’ to meet the right hand. My question, finally, is how would I move the weapon I have to follow the other hand without any major snapping? In maya, I’d simply switch the affecting parent to the left hand from the right, but how can I achieve this most efficiently in Unreal Engine?