Unfortunately it is too late to do anything but, take what Regent books for you. Depending on when in August you are departing, your flights should be available shortly as generally available approx. 75 days out.
Based on the above letter about custom air, you are probably too late to deviate although never hurts to try as if you can't get what you want, it costs you nothing but, appears the time fraame is such that there will be a lot of waiting. With all the flight issues and price increases unlikely you will be able to change whatever Regent provides which likely will be the least expensive routing for them.
Gook luck and have a great time.