you may have problems getting them to accept the routing as a single trip as a stopover is normally a stop as part of your route
you currently have DUB-MCO and DUB-EWR-MCO is a valid routing
you want to change to DUB-LAX and DUB-EWR-MCO-LAX is not showing as a valid routing, there are a number of DUB-LAX options but none show as via MCO
In theory if they would accept the routing it would be taxes plus change fee, you can buy a revenue ticket for your date on United for £103 (direct, no need to go via IAH)