The core point is the same: With sufficient test coverage, you can change one variable (Python 3 vs 2) and control for all others. Tests are not an end, they are a means to an end; the courage to change your software in the face of a constantly changing world.
Sounds like you're not there, but maybe you have specific domains of EVE that you feel comfortable changing at your whim. That's the success to focus on.
Somewhat naive too to be honest. As someone that had just plain old ruby circa 1.8.5ish code (not rails) that I ported to 1.9 a while back, even with tests there were differences in behavior that I guarantee you probably weren't thinking of writing a unit test for.
Unit tests are nice, but they don't make things braindead easy. Just make it a bit easier to narrow down where something might be broken/different.
Sounds like you're not there, but maybe you have specific domains of EVE that you feel comfortable changing at your whim. That's the success to focus on.