I think that any harpist who comes to you because of technical problems has to be willing to change something to fix the problem. It’s not necessarily a matter of changing technique or changing methods.
About 10 years ago a professional harpist(and a very good one) came to me and asked me to overhaul her technique. She had had a total of about 5 years harp instruction many years earlier, and irrespective of the fact that she had studied another school of playing, her technique had devolved into an awful mess that bore no resemblance to the method she had initially studied. After consulting with two very fine teachers of French method, I decided that the best solution was to make the major changes necessary. I was afraid that if I only tried to tweek what she was doing, we’d end up with a technique that I didn’t know would work or not. When she came for the first lesson I laid down some rules. I said I would do what she wanted ONLY if she agreed to these rules. I said that she had to promise to come for a weekly lesson no matter what, even if she had not had much or any time to practice, and this would be for a minimum of 6 months. Since she was a busy gig harpist, i told her to go ahead and do her bread-and-butter wedding gigs, but under no circumstances was she to take on difficult orchestra parts, concertos, chamber music, etc. for at least 6 months. And I told her that she had to agree to do exactly what I said.
She was a model student. The core of our work in fixing everything were the 40 easy etudes of Bochsa(on the advice of one of the two teachers I had consulted). She loved working on those etudes and to this day uses them to brush up when she has the time. I gave her a series of easy short pieces to make the transformation, and told her initially to just go through the motions correctly and not worry about anything else(tone, musicianship, etc.) until the new hand and arm position, finger motion etc. felt comfortable.
It took about 3 months for the new way of doing things to settle in, and several more months to apply it to the more advanced repertoire she was used to playing. Her sound became fuller and she had much more control over dynamics. I believe she was in her early 40’s when she did this, so I don’t think there is ever a time when one can’t make changes. But the student has to be willing to make them, and the teacher has to be very careful.