The change fee is liekly $200 per ticket (but that will be in the fare bucket conditions). In addition, you will pay any fare differences.

The best way to handle this is not as a series of hypotheticals, but to actually price out each option and use the cheapest option.

First, price out the new itinerary you need. That will yield a $ cost.

Then look at what happens if you use one ticket or the other (value of ticket less $200) or both tickets (value of the two tickets less $400).

The lowest net is your best option.