It depends on what you consider an upgrade. I don't think the overall sound is better due to compression, and, some of the features in the Audya, like sub outs, unlimited program registrations, and 2 mics, aren't present. OTOH, we are talking about a new and modern OS, with new features, large color touchsceen, and new styles for the younger generation(IMO). If I didn't have my Audya 76 AJAMSONIC, I would probably have to have an SD90.
According to one of Ketron AJ‘s posts some weeks ago, the new models don‘t have compressed sounds in terms of smaller sample size, but in terms of compression as an effect added to the sounds. Anything else would be a ridiculous move given the general development of sample sizes, the price segment of ghe SD9 and the age of the Audya...