public Boolean isRevertAllowed() {
    return revertAllowed != null ? (revertAllowed != 0) : null;
  }
Gets whether this upgrade supports being reverted. Upgrades can be reverted
(downgraded after finalization) if they are either
{@link RepositoryType#MAINT} or {@link RepositoryType#PATCH} and have never
been previously downgraded.

@return {@code true} if this upgrade can potentially be revereted.
ambari-server/src/main/java/org/apache/ambari/server/orm/entities/UpgradeEntity.java