I think the easiest method would be to download the configuration file and do a search and replace for the outgoing interface in the gateway.
The configuration can be downloaded under:
Configureat--Update--Config File
This is also where you will apply the revised configuration with the replace option.
The Gateway command statement sample is like this:
set ike gateway "GatewayRemote1" address remote1.site.com Main outgoing-interface "ethernet0/1" preshare "K8cYqf+6NFYhCoHjw2J8nulQvn5Q==" sec-level standard
You would use search and replace a unique pattern from say outgoing-interface through preshare
outgoing-interface "serial1/0" preshare
to
outgoing-interface "ethernet0/1" preshare
Double check the matching pattern with your file. Then confirm the file looks good by using the compare file function in your word processor. This will highlight all the differences so you can see that only these commands changed.
Then upload it when you move the firewall with the replace option.