Sandman is correct. We are running both versions. Also, you should carefully verify each module you use from the new version, and make sure you are running the version latest version of the software. There have been many bugs in version 6 since it's release. They have been doing a pretty good job fixing the bugs as they come out, but still a good idea to make sure you are getting the response you think that you should be getting.
We don't use Enercalc, but this situation is absurd. I can see not offering support for older versions forever, but the latest version of ALL software should be able to open files created on earlier versions. Lazy programming doesn't make me want to start using them anytime soon.