I am not an expert in developmental genetics but I would imagine you
could replace nearly all genes with human (or otere primate) and it
would still look the same. I am under the impression that when and where
a protein is expressed is more important than the actual coding
sequence... Since so many of the proteins are conserved. I would bet
that if you changed all the upstream promotor sequences or were able to
modify various rna/protein gradients you would have a more profound
effect. Just a guess.