The pig whipworm eggs used previously for this purpose can hatch in the human body but they cannot survive, so no foothold therapy is needed. Instead, they're just enough to attract the attention of a perhaps overly-excitable immune system.
However when crossing hosts signals can get crossed and they can cause serious health problems like eating your retina, brain, hearts, etc. Be very careful with parasites that aren't evolved for humans.
Necator Americanus was always specialized to infect humans. But it evolved to infect Africans, who brought it to the new world. For blacks, it doesn't pose much of a problem. For whites, it does.
If this became a routine treatment, it seems pretty likely that they'd start being able to survive.
For something to evolve like that you need to re-introduce the mutated strains back to the source.
If there is no re-introduction (which there wouldn't be, because the worms would be bred in an isolated environment and not the sewer system), then there isn't really anything to worry about. There is no selection pressure at the point where breeding and distribution occurs.
I imagine if such a thing did occur, they would just kill the batch they were breeding and start over from a known strain.
(Not a microbiologist, so who knows though)