In the meantime Hortonworks has reflected this limitation in the upgrade documentation:
Ambari 2.7 only supports fully managing a HDP 3.0
cluster. If you are running any other HDP version, you must first
upgrade to HDP 2.6 using Ambari 2.6. You must first upgrade to HDP 2.6.5 if you
use Hive. Then, upgrade to Ambari 2.7 and use it to upgrade your
cluster to HDP 3.0."