I have the same question, I feel that the link above does not answer my understanding of the question.

To be clear, I think we want to know, come Myst release. Will we be able to keep our old worlds and evolve them into Myst or will we need to create a new world when the next expansion releases ?

The only thing that kinda comes close to answering the assumed question is as follows:

"Is it required to start a new save on the Public Test branch?
No! You can simply continue with your normal Early Access save. However, please note that new content does not appear in already explored/generated areas. There are some important other things to take note of before switching versions as well (read the three questions below)."