Element 3D V2 will replace V1.6 when you install. Projects created in V2 will not open in V1. (Of course, we will always make older versions of plug-ins available if you need them.)
We worked hard to get projects to convert properly but our goal was to modernize the internal system. So the new coordinate system and anchor points work better but if you changed the anchor point in a previous project, you may need to adjust it again. Same for SSAO, we improved the algorithm (and got rid of some nasty banding) but some cases might look a touch different than before.
Of course, be sure to back up your V1.6 projects before opening them in V2, that way you can at least install the older version if something doesn’t look right.
With that said, be careful. If you don’t need to convert the project, why risk it. Just finish it in V1 and then move on to the glorious V2!