Here's a quicky diagram, irreverently in my best 'Chingrish'
(in kind jest.. ;P to show 'how to doo': http://www.photonlexicon.com/forums/attachment.php?attachmentid=38670&d=1370973403
…point being, since an 'X' really does not need all the 'translation' (length) of a Y, if both mirs are-of equal length
(vs, ie: CTs, where the X is already a short, 'stubby' box, compared to the elongated, taper-cornered Y..) then you can do what is being proposed in the 'sex change' graphic, above, and simply adjust the
(former Y, now, 'X'..) galvo in the mount, to account for the shortening-up, and then, re-tune both amps to play well together, again.
Here's more seasoned-advice on how to change your Galvo-Mirs: