Swans

I don't have exactly the same experience, but I have become very close to one of the 2 dominant mating pairs we have at Poole Park over the last 6 years. If they are out on the big lake I call and when they hear they look for me and then come as quickly as possible, that's the pair by themselves and when they have cygnets too. I've also seen how ruthless they are dealing with the other swans there. Not only can they drive the herd, anything upto 20 swans, but they can also drive the other dominant pair off too. The two pairs basically share the big lake between them and the herd spend their time trying to avoid them.
I know from my interactions that trust is a major factor for them, but from watching the interactions between the herd and dominant swans, there is definitely a heirarchy, a few that rule and the rest have to be submissive and run.
As sad as it is, it sounds like your swan has to accept her new role as she is no longer part of a dominant pair, she no longer has territory which means she probably has to join a herd somewhere for some kind of safety. At least that way she has a chance of finding a new partner, depending on her age, and maybe she gets another chance of regaining the pond where you know her from or some new territory. But to do that she will have to do exactly what that new pair are doing to her now. It's how they gain and maintain territory. All the dominant pairs do exactly the same thing.

Oh, I forgot, the other thing many people I know haven't seen is that the dominant mating pairs tend to be as equally ruthless with their own cygnets. I've watched it happen with every set of cygnets the mating pair I am close to have had over the last 4 years. It gets to a point where the mum suddenly starts attacking them and then dad joins in and they are driven off. I'm still not sure if it's related to size/age of their cygnets or if it's simply because they want to have new ones and want their territory back for themselves. Maybe it's a combination of all.