Can I upgrade to CR 6.3 in LR 2.7?
No. Lr does not access the Camera Raw plug-in, instead it includes it's own internal version. However, you could convert your NEF images using DNG Converter 6.3 and these will open in Lr 2.7
ACR 5.x is for Photoshop CS4 and the RAW engine is in sync with LR 2.x
ACR 6.x is for Photoshop CS5 and the RAW engine is in sync with LR 3.x.
This is how it's been since ACR 4.x and LR 1.x.
"In sync with" means the RAW engines are the same so LR can hand off a file to Photoshop and have ACR open it with the same set of adjustments w/o rendering to an intermediate raster format file.
If you use both LR and PS, it might be better to keep LR up-to-date with your cameras and only upgrade PS when you feel you need to, because by using a TIF or PSD intermediate file you can edit in LR and pass off to PS. You can't do the other way around if you are wanting to support a new camera.
I have also purchased the Nikon D7000.
Downloaded and installed LR 3.3RC. Also downloaded and installed ACR 6.3R1.
Both work with the D7000 NEF-files with ONE exception. If I shoot a "Multiple Exposure" (one feature of the D7000) the resulting image in both LR and ACR displays in "pink". All other images display correctly.
If I open the pink images in Nikon Capture NX2 2.2.6 they open correctly.
If I use the White Balance tool in LR i can bring the image to "almost normal" but the WB values are Temp: 2150, Tint: -138. Conclusion: the Multi Exposure Image from the D7000 is completely mismatched by LR/ACR. But they are correctly matched by Capture NX2.
This obviously needs to be corrected before final release.