Current word on the general availability date for NAV 2009 is 12/1/2008. After this date, the only new licenses that will be available from Microsoft will be NAV 2009. The version includes both the classic and the new role-tailored client, and the code base is theoretically identical to version 5, Service Pack 1. This applies--of course--only to the classic client.
Unmodified reports, forms, etc., will require no data conversion, modification, etc. (because you'll just use the NAV 2009 version of the code). Modified forms will need to be put through a conversion process to appear as "pages" in the new client.
Although the conversion tool can be run with minimal effort on an existing form, to make the new "page" really usable will take several steps. My guess is that future versions of NAV (perhaps a service pack?) will automate and combine these steps. They just seem too cumbersome to support the development process.
Some developers are already developing their own tools to handle some of the more time consuming steps. Eventually Microsoft will turn attention to this.
The good news is that the new client looks really good. In this release, I think most businesses will run both the classic and the new (role tailored) client.
