Here's my situation: I want to fly LAX-SEA-LAX in F. I was going to buy two Economy tickets at USD 69 each and then upgrade each one of them against payment of USD 100 per leg.

Now I stupidly waited too long to buy my ticket, and there's no more USD 69 fare available for SEA-LAX. But who knows, maybe it'll open up again.

Thus, to hedge my options, I was going to buy LAX-SEA in Y at USD 69 and SEA-LAX in F at USD 309. Then I was going to upgrade LAX-SEA against payment of USD 100.

If the cheap Y fares do not open up anymore on SEA-LAX, tough look, I'll have paid my F flight. Total bill 479

However, should the cheap Y fare open up, I was going to cancel the paid F ticket, and with the credit of USD 309 I was going to pay for a cheap Y ticket (USD 69) on that same flight and two upgrades (USD 200). That would leave me with a remaining credit of 40 USD which I don't mind (NB: I try to avoid too much credit as I am not sure when will be the next time I'll have the opportunity to use VX, I don't live in the US).

My question though is: can I cancel my return leg in F and re-book in Y on the same flight, thus benefiting from a cheaper fare? Is there a penalty involved in canceling/changing the F fare? Basically what I don't know is whether returning my F fare and flying on the same flight in Y is considered a cancellation or a change. And I don't know if it's possible to cancel the return leg without touching the outbound leg.

I do know it is possible to change from one Y fare to a cheaper Y fare - but can I also change from an F fare to a Y fare on the same flight, and to all of that for just one segment of my trip?

Thanks for any hints.