I do not think that is possible because once you run your app, android will prompt the user if they want your app to be the default SMS. If the user says "Yes" then your app will handle ALL sms messages from that point on, even after the user exits your app.
The only way (that I know of) to switch the default sms app back to the original sms app is for the user to re-run the original sms app, then android will re-prompt the user to switch it back to that app.