Hello Steve,
You can have both versions installed on the same computer, just move the installation directory of the older version to some custom location, before installing a newer one to the default location.
As for the problems that you are observing, it would be helpful, if you can provide an example of the image set that can be properly aligned in the version 1.2, but fails (or produce wrong result) in the versions 1.4, 1.5.