This is a fairly interesting presupposition. I think that it should be noted that like Stalin's relationship with Lenin as he consolidated his power, there would be a very uneasy state between Napoleon and Old Major. I think that the former would do everything possible to silence the latter, constructed "accidental" death not being excluded. For his part, Old Major would not abandon Animalism, but rather suggest that the pigs have embraced too much of the ways of the human. I don't think that Old Major would repudiate Animalism. However, it would be interesting to see how he would have reacted to the fact that it was his own pigs, his own kind, that strayed so far from the tenets of Animalism because of their love for power. Old Major would point out how the fundamental problem was not the equal treatment of all animals. In his basic treatise in Chapter 1, the notion of a classless society was advocated, one without distinction or higher or lower "classes." This becomes where the pigs and dogs violated Old Major's ideas and it is probably at this point where Old Major would point out that the experiment on the famr is not really Animalism, evident with the ending of the novel where the farm is renamed and "no difference" is evident between pigs and humans. This might be where his first line of critique lies in that Animalism is not really rejected by the farm's experiment, but rather that the fault lies in how the pigs established "classes" of animalsm, moving away from Old Major's vision.
Old Major's teachings are used to form the original Commandments of Animalism. His view of humanity as upper-class oppressors to the animals, who he sees as equal to them and capable of forming their own lifestyle, is absolutely essential to his Utopian ideals.
"All the habits of Man are evil. And, above all, no animal must ever tyrannise over his own kind. Weak or strong, clever or simple, we are all brothers. No animal must ever kill any other animal. All animals are equal."
(Orwell, Animal Farm, msxnet.org)
Had he lived to see how the pigs subverted and changed his teachings, he would have pointed at their hypocrisy and failure to live up to his ideals. His idea that no animal should adopt human habits is rejected by the pigs entirely, as they start to live in the farmhouse and drink alcohol; when the pigs change the law from "All animals are equal" to "All animals are equal, but some are more equal than others," Old Major would probably reject their leadership outright, since it directly contradicts his central ideal. It is likely that he would be outspoken by Squealer, or put to death for heresy, but his staunch belief in the true equality of animals would not waver, especially as he would see how the animals now need to work harder than ever to support the pigs.