Is it possible to change the save as project on exit to save as mp3 or other filetype ( easily changed by user ) ?
In short, no, Audacity will always prompt you to save a project.
The only formats that save all of the project information are Audacity Projects, and “Lossless Copy of Project”.
Exported audio files have the audio data, but cannot save envelopes, time tracks, labels, separate clips, un-mixed tracks … and depending on the export format, may also reduce the sound quality. The developers therefore consider it essential to ask the user if they want to save the project before irreversibly discarding the project.