Why should it mean a new Vortex?
The Vortex isn't part of the TARDIS, it doesn't change when the TARDIS does. Changing the look of the Vortex makes no sense, unlike changing the TARDIS, since the TARDIS is alive and can change it's self. I'm wondering if the TARDIS will change as the Doctor Regenerates.
And the main reason that they want the set changed is because the current one is falling apart, the buttons and stuff keep falling off. HOWEVER, if they are going to redesign the interior (Most likely) or just rebuild the current design, I have no idea. Hopefully, they'll just be rebuilding it.
Also, casting a not very well known actor was probably done because they won't have to pay him as much. Meaning that hopefully they'll put more money into set design and effects so they can leave Earth more.