The drawback of making it a single dialog box in the user experience is that the "current name" changes when you use "Save as...". So if you do a "Save" next time, you save it as the exported name and format instead of the original one. If, for example, your export format is .png, and your original format is layered, this could bite you. Also you keep flipping between two names.

Sometimes an "autoexport on save" feature would be nice. Export it to the derived format as well as save the original file with one keypress.