I recently upgraded a 15.x installation on Netgear WNDR3700v2 to "LEDE Reboot 17.01.4 r3560-79f57e422d / LuCI lede-17.01 branch (git-17.290.79498-d3f0685) "
It went well - until I tried to restore settings from a backup that I had made from the 15.x installation.
While it appeared ok there was a significant issue - dnsmasq failed to start because of user:group problems.
I think the issue was probably that 17.x requires a user:group of dnsmasq but 15.x was using root - so it failed to start because user:group was not there.
In the end I reloaded the firmware and redid the configuration by hand rather than trying to patch things.