Not at all. New laws would have to be passed, which would need to be approved by the Council and Parliament.
It would be pretty far-fetched considering the current scope of the Union, and it's a political mine field with a not too obvious pay-off. It might, however, be necessary if there are developments towards an European defence pact.
What would be within the scope of the EU is to make the standards for entrance stricter, but this wouldn't really affect the countries already in the Union or prevent backsliding.