New packages of lightdm and lightdm-gtk-greeter are now published as 64-bit binaries.
Migrating from 32-bit to 64-bit without a reboot is a not scenario handled by lightdm: if you log out immediately after updating the package, reloading the binary registry will fail and put the service into maintenance.

Therefore updating to a new BE is recommended.

If you log out right after the update and your login manager does not restart, clearing the service with:

svcadm clear lightdm

is enough to get the 64-bit version to reload properly.

The list of services put in maintenance mode can be inspected with:

svcs -xv