You can update them all in one go.
1) Create a smart list to show the items you want.
e.g
2) Select all the items returned*
3) Right click and select Tag
4) In the filename tag enter =Replace([Filename],Y:,X:) and then press return
*May be just select one file to try it on first, then when you are happy it is doing what you want select all the remaining files.
Don't forget that there is Edit>Undo if it goes wrong, or better stillmake a back up of your library before you start.
