Graphics card DirectX backwards compatability

The reason why DirectX 10 cards cannot be upgraded to DirectX 11 would most likely be to do with DirectX 11 mandating particular features such as a particular level of precision in mathematical operations, that kind of thing is difficult and a bit annoying to back-port onto hardware that was not designed to work with it.

This would be the same as a particular CPU not supporting new features introduced by a newer CPU, sure the new features could be emulated (and may run not too much slower than a native implementation) but the CPU itself still will not truly support the required features. A similar idea might be the possibility of emulating SSE instructions on a non-SSE processor, sure you could do it, but it's a lot of mangling and would be a lot slower than a full hardware implementation.

You may well, with custom software, get a DirectX 10 card to look as good as a DirectX 11 card and to do almost everything in a similar fashion, but all the native features or extra precision that DirectX 11 mandates would be missing and in general I think the software/emulation layer would be quite a bit slower than a full hardware approach.