Because of the difference in sensor size there will be no option to use the lenses intended for those with the 6D, at least not without vignetting or an adaptor.

If you are thinking of upgrading then also consider the Nikon D600. Neither of these have been reviewed yet.

Thankfully I think pretty much all my lenses are EF and not EF-S

would you be better getting a used 5d mk1 & see how it goes? or even a cheap  FF film camera?

I'm not seriously considering this by the way. The 7D suits me fine. My one and only gripe with it is that it tends to be quite noisy at anything above ISO 200.

Not the most exciting video but it might answer a few questions.

Same as the D600, both are a bit odd really, probably lovely cameras but when you the prices settle down how much different is it to the next model up.  The D600 is £1725 according to camerapricebuster but the D800 is only £2124 which isn't that much more really.

The 6D is just listed at £1799 so far, the 5dIII is £2469, bit of a jump but not too far if your willing to spend £1800 on a camera then £2500 doesn't seem that much further away.

http://camerapricebuster.com/Canon_EOS_6D_Body_pc.html