After 25 years of flying BA, it's time to trigger my first 2-4-1. The dates for the booking window are about to come out, and given the general lack of availability, I want to ensure I maximise my chances. So I need a bit of help from the 2-4-1 experts out there.

I am planning to fly into IAD and back from DEN two weeks later, in J or F. I believe it is sensible to seize the IAD flights when they are available, and then add on the return flights from DFW two weeks later when they become available too.

So is it the case that I book the outbound flights as a one way, using the voucher? Is it OK to do this online? And then two weeks later I phone up BA and ask them to amend this to turn a one-way into a return, adding on the return from DEN, for a change fee (which may be waived as I am gold?). In other words I can book the 2-4-1 as a one way, and amend it by phone two weeks later to a return from a different location (and potentially in a different class, J v F)?

Or do I have to book it as a return from the start? I assume this would involve booking the IAD inbound and DFW inbound on the same day? And then amend it two weeks later?

Apologies for what must be rookie questions to many of you.