Changing graphic extensions PNG to TIF only in the display

We have files that use PNG files on the server, but the writers only have TIF files on their PC. I have a simple function:

edit -ok -untagged "$docname"
substitute -a -wrapscan /.png" width="1.27cm"/.tif" width="1.27cm"/

To swap it out, but then I have to swap it out again to save it for the server.

Any ideas?

Dean