>>I could reattach and all, but we want a way to change the property with xref in place, <<
That capacity is available in the XREF dialoag box (has been since R14 at least). Just dbl-click on the word OVERLAY in the xref dialog box and it will toggle to ATTACH, dbl-click to toggle it back.
Currently I'm unaware of a switch usable in a script or lisp function, but I'll look.