We can always migrate posts content, without migrating users (as content was already publicized in... well, public.). At least in EU it works and is legal.

/Estel

// Edit

Of course, it would be only about content from users, that are not interested in giving permission (can't be pinged, or not willing to).

// Edit 2

In worse situation, we may do something like our own "wayback machine", i.e. big archive of "snapshot" from content of threads presented already, ending with link to actual, "live" discussion in future forum.

Not most convenient, but there are tools to do so via batch, and it's better than nothing.