Excel 5.0 Dialog Sheets

These were replaced long ago with User Forms, but were left with backward VBA compatibility. I have one left that I hadn't thought about since then until it just stopped working yesterday. Running a macro with the VBA line "DialogSheets("Input").Show" no longer pops up the dialog box named "Input", but gives me an undefined VBA 400 error. If I run it from within VBA, I get "Run-time error '1004': Show method of DialogSheet class failed." Does this likely mean Microsoft has ended the backward compatibility? If so, is there a way that I can convert the dialog sheet to a user form without having to manually rebuild the whole thing?