I have a tablet where when attempting to install 1709 (ISO) from 1703, the message in the subject line is the end result, preventing the upgrade from getting past the preliminary stages. Is there a way past it? I did see the "AllowOSUpgrade" Registry change, but it had no effect (I think it only pertains to getting new builds to show up in WU). The tablet is Clover Trail-based, which in my case does work properly with 1703, and is also a build that supposedly didn't have support but was in fact the first build since early W10 builds to work properly. (1703 later was also blocked from installation on Clover Trail, but I installed it very early on, apparently before the block was put in place.) I strongly suspect that if I were able to actually install 1709, it would also work.