I can't see the dialog Box that you are referring to, so I don't know how
you are attempting this. Can you post a screenshot somewhere, or describe it
in more detail?
As an exercise, I opened a site in SPD. I then copied the EditForm.aspx file
for a list and renamed the original to EditForm_orig.aspx. I then modified
the EditForm.aspx and saved the changes. When I refresh the page in the
browser, the changes appear. Both files appear under the list in SPD, and I
can see both in the List -> Properties Dialog box on the Supporting Files
tab. I didn't need to reset the server as these were simple CSS changes on
the page.
I tried this with "Contributor Settings" enabled and disabled, but didn't
see a difference.