Happens to a lot of lurchers over time. I have found some stay soft mouth for years then all of a sudden go hard mouth instantly.
Some are hard mouthed from the start.
I also think it really has nothing to do with some of them taking foxes pre-ban as some people say.
Once they turn hard mouthed, they wont go back, just the way it is in my opinion.
It also all depends what you prefer, however, if a dog is catching well and has a high catch rate ect, it would be a pity, in my opinion, to move it on just for that, as a lot do. !