I just inherited a web site that was created by a designer. The site was originally created with all *.html files. The designer renamed all the *.html files to *.aspx files.
To save yourself from the drama of manually editing the project file like David Frette details, I suggest you remove the file from the project and create a new file with the same name with a code-behind. Then copy-paste the contents of the original aspx or ascx to the new files.