How to inhibit save, allow save-as?

[G Lykos]

Greetings! From within a module, with the user already having opened a
workbook, would like to inhibit the user being able to save back on top of
the original file since I'm modifying data, but allow the user to save-as
under another name if desired, all to occur when closing the workbook.

Imagined sequence would look like: as the user closes the workbook
following changes, is asked via standard dialog box if wants to save
changes, and/but if so, then proceeds to save-as dialog box prompting for
file name.

Thanks in advance for ideas!
George