If your setup relies on Network Address Translation (NAT), you must make sure that there is a set of outbound NAT rules for each OPT_WAN interface. If you have Outbound NAT set to Automatic outbound NAT rule generation or Hybrid outbound NAT rule generation, no worries: the necessary outbound NAT rules will be generated automatically. If you have it set to Manual outbound NAT rule generation, however, as would be the case if you have a CARP configuration and therefore must edit the rules in order to direct traffic to the WAN virtual IP, then you will have to create your own set of outbound NAT rules for each OPT_WAN interface. In such cases, you can use the existing rules as a template for the new rules with the Copy option; just change the Interface setting to the new OPT_WAN interface and the Address to the OPT_WAN virtual IP address. 

Also, if you have any 1:1 NAT mappings, keep in mind that these mappings specify a unique WAN interface. Such 1:1 mappings also override any other outbound NAT settings for the interface on which it is configured. If you want the 1:1 mapping to work on the new OPT_WAN interface, you will have to add an entry for it.

If you have any Port Forwarding entries, take heed of the fact that, as with 1:1 NAT, these entries specify a unique WAN interface. If you want to open a port on multiple WAN interfaces, the easiest way to do this is to use the Copy option to create a new rule based on the old one. Change the interface to the new OPT_WAN interface; the Destination will automatically update to OPT_WAN address when you do so. Update the Description if desired. Then click on the Save button and when the page reloads, click on Apply Changes.