Yes, it is true that HS3 will replace any occurence of HS2 in the older projects. The existing automation will not get lost but I’m not sure that the total backward compatibility is guaranteed. In the worst case, some parameters can be left unassigned or wrongly remapped. So, if you want to be completely safe I would suggest to wait for the trial version and test HS3 on your particular projects.
Best regards
Miloslav