The only way I know to change the path of an XREF is to go into the XREF Menu and do all of them one by one.
You select the XREF you want to change the path for, go down in the "details" area of the XREF menu, go down to "Found at", select that, click on the little dots on the right, and give it the path you want.
As long as the path is correct the file for the xref should be found because when you change XREF paths this way, you are prompted to browse to the actual file, itself. Not just the folder it is located in. After you tell it where the file is at, it will always find it unless it is moved or the folder containing the file is renamed.
One option might be to use the "Autodesk Reference Manager" in autocad which is a standalone application. To use it you would browse to:
start> all programs> Autodesk> Autocad> Reference Manager
Reference Manager is perfect for changing the XREF paths in multiple drawings.
The help menu (Autocad 2010 at least) sais this:
"When you relocate drawing files or the external files that they reference, including other drawing files, text fonts, images, and plot configurations, the reference paths that are saved in the drawing need to be updated. The Autodesk Reference Manager lists referenced files in selected drawings and provides tools to modify the saved reference paths without having to open each drawing file. You can also use Reference Manager to identify and fix unresolved references."