When a user equipment device (UE) is served with standalone connectivity by a first access node on a first carrier, the first access node could detect a trigger for transitioning the UE from the standalone connectivity to dual connectivity. In response to at least the trigger, the first access node could then (i) transition the UE from the first carrier to a second carrier selected based on the second carrier being lower in frequency than the first carrier, so as to help facilitate quality communication in the dual connectivity service, and then (ii) transition the UE from the standalone connectivity to the dual connectivity.