oi_151a_prestable9 AKA oi_151a9 is a bug and security fix release with another JDS rebuild. This is not an ISO release.
The python package issue for a8 should be fixed in the repo now. This means if you got a pkg error trying to upgrade to a8, you should try updating and should have more success now.
If you have updated from oi_151a8, the package gvfs will be missing. Check this with “pkg info gvfs”. This package was inadvertently deleted in oi_151a8. Without this package, desktop mounting and unmounting CD and DVD disks and USB drives will be incorrect. Install this package with “pkg install gvfs”.
See Release Notes.