If Encore 6 does not open Encore 5 files, what’s the point? Safe to say this needs to a primary function of v.6. No idea about Encore 4.5.5 and earlier.
I had a Mac G4 set to dual-boot in OS 9/OS 10.4.11 Tiger. Encore 4.5.5 could open 4.0.2 and earlier in OS 9 and save. I could then boot into Tiger, open the 4.5.5 files in Encore 5.0.2 and Save. Now I could transfer them to my iMac and open those in 5.0.5/6/7 in MacOS 10.7—10.14 Mojave. Convoluted? Hell yes but it worked. Let’s hope we don’t have to go through that again.