you cannot change on the fly the routes advertised in OSPF by core backup given the link state nature of the protocol.
However, you can increase the OSPF cost of the links betweeen (core backup, WAN edge active) and of the link (core backup, WAN edge backup) by using ip ospf cost 50 at both ends of each link in interface configuration mode.
This makes core backup less preferred by the WAN routers then core active, because of the cumulative nature of OSPF metric in a single area (area 1) as in your topology.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...