Unrelated problems, really. Having written such translation library, I can say with confidence that the only reason for this is lack of interest in it.

Graph to graph conversion can be tricky due to subtle differences in implementation (even between different versions of the same framework), but it's perfectly possible, though not many utilities go all the way to graph rewriting if required.

Problems arise with custom layers and lambdas, which are not always serialised with the graph depending on the format.

Human brains have high degrees of plasticity -- our brain is much more generic than its usual functional organization would suggest. I don't think we'd be able to upload brains ("state vectors" was the sci-fi buzzword) before digital supports are able to emulate that.