kde 3-4 question

[Mike C]

At present the majority of F8 kde users are using the default kde 3.

When the time comes to upgrade to F9 does anyone know if the desktop for existing
user areas will automatically use kde 4 after an upgrade?  If so does anyone
know if there are apps that will fail if the user setup is asis from F8?

Or will some re-configuring be needed for the transition from kde 3 to 4?