I have been looking for this for a while and found no real answer on this forum, but i figured it out on my own, so im making a guide!
the problem: when you change an xbow into a bow with MES, the character keeps holding the bow in the wrong hand!
its actually really simple:
first download a hex editor
(Xvi32 works great and is free)
you extract it anywhere, and then when it opens, go file -> Open
and locate the Itemcache.wdb
now you should find your weapon on wowhead or some place, you have to copy the name of the item you want to change the hand for, i for example did: i changed my gladiators xbow into the veng bow in MES, then i opened itemcache.wdb in Xvi32 and then you have to go to search and choose text string and search: Vengeful Gladiator's Longbow
its case sentitive and you have to make sure there are no extra spaces or anything
now it should warp you to a line somewhere in the text, then in one of the top 4 lines, the hand position is, its either 1A (for xbows, 1A is the right hand) OR its 0F (left hand for bows) all you have to do is switch it.
example:
you are switching from an xbow to a bow you have to search for the name, and find the hand handle, since you're switching FROM an xbow, you have to look for 1A, when you find it, push it, and then push 0F on the keyboard, its that easy!
enjoy your bow/xbow being in the proper hand =)