I've updated one system from 2.9 Beta to 2.9 official version and another production system from 2.6.2 (.19) directly to 2.9 without issues. Both upgrades run smooth.
one important notice - i have a lot of 1800v2 APs and all of these are not automatically serving clients after the upgrade. From the status page everything looks good - means firmware version is correct, status shows "up to date" and radios are online but not a client connected to these APs. A manual AP reboot was required to fix it.