I take it for granted that Encore6 will read/import Encore5 format files (and corresponding MTDL 4)…

But I haven’t seen a commitment that it would do the same for earlier formats, mainly E455/M355

I see two reasons why it is important:

1. When Encore 5 came out, not everybody was enthusiastic about it. Some elements improved, others got more difficult. Perhaps not all Encore users have a E5 license, nor use it nowadays for current work.

2. Even when migrating to E5 for current work, most of us must have files created (well) before E5 availability, sometimes a very huge library of scores they never had to update later on.
My own reason for that is that I left a Belgian choir when moving to France, 15 years ago, to join new ones with completely different repertoires.

I think we should not be forced to convert the very old libraries, that we might want to re-access some time in the future.
Taking into account that the format differences between E4 and E5 are rather small, considering the E4 functionality.