We already several major updates to our earlier sceneries:
1) Zurich has been updated, for free, when a new terminal got replaced.
2) Chicago has been updated, for free, when a new runway has been added. It will be updated again in the near future
3) JFK has been updated, for a small price (because the rework was massive), too.
And we already said here on the forum, the next scenery updates will probably be KLAS first, and KORD next, and we'll start working on them after KMEM will be released.
These sceneries will have to be updated first, because they are even older than KDFW so, as a general rule, you should expect we'll update older sceneries first, which means KDFW might come, as an update, after KLAS and KORD updates.
So, the real answer to your question, should be:
As long FSDT will stay business, you can expect ALL our sceneries will eventually be updated every few years, either with a free or a payware update, because we'll have to keep them "fresh" on the market, and to adapt to new hardware or new versions of the sim, and update the airport to real word changes in addition.