I suspect your question is, whether your place in the pecking order of seat allocation changes if you switch between carriers on a flexible ticket. I leave those with greater knowledge than I to answer that.
However, I concur with slim_slag that the fun starts when you start to push the envelope as to what you can actually practically do with a ticket. I have found, for instance, that in Asia, it is usually possible to turn up for a flight other than that you are ticketed and booked for and you will get on with no extra penalty, as long as it is in the same class on the same airline and the flight is not full (I know this is a bit naughty
). This works regardless of whether the ticket is discounted or not.
That said, conversely, in Asia it can be difficult to endorse even a full fare ticket from one airline to another, sometimes requiring the issuing airline to physically endorse the ticket over. Difficult if out of working hours!
Appreciate that in Europe and US, things are different.