nwiki will arrive with Moodle 2.0. And Moodle 2.0 *requires Moodle 1.9* installed in order to upgrade.
I guess the best migration path will be to continue using the glorious old wiki until 2.0 upgrade. Of course there will be a long beta stage, before 2.0 release, and that'll be the time to test and report about such wiki upgrades.
Right now we are working hard against Moodle 1.9 to polish all those details that never are detected before release, so we haven't analysed yet how the nwiki refactoring (changes) will be, and what parts of them will be modified. I hope we won't need to modify the DB layer, that will be good news from a complexity (and upgrade) point of view.
Obviously, people using nwiki with previous versions of Moodle should be able to upgrade transparently too... but IMHO I'd continue using the old one if upgrade ASAP isn't a must for you.
Ciao