If you are upgrading directly from a Classic Castaway version (v2016.3 or earlier), you do not need to uninstall. The installation process for Castaway 2019 will automatically remove earlier versions.
If you have the public Beta version, the Castaway 2019 installer will not remove the Beta version. It can be happily co-installed, but we do recommend removing the Beta version anyway, simply to reduce the chance of confusion between the versions.
Files created in earlier versions of Castaway will work immediately in Castaway 2019,. You don't need to do anything, just open them up and start working.