Most of my clients are professionals and work in positions such as business analysts, accountants, teachers, IT professionals or engineers. They all are very skilled and have been employed because of their professional knowledge. Their English has reached a high level of fluency even though pronunciation errors still occur.
Pronunciation training can change all this for a while. I always forewarn my clients that there will be a period when they feel they are getting worse before they get better. This is because they are becoming aware of their 'mistakes' and developing self correction strategies which of course interrupts fluency.
This is part of the process and will change in time. However it can be very frustrating. I also tell them that they are very skilled to be working at such a level bilingually but unfortunately pronunciation training often focuses on what they can't do rather than what they can do. It will often seem as they are still doing many things 'wrongly' when in fact they have made huge improvements in their clarity of speech.
As I said before, it is a continual and very gradual process and change will not happen overnight. My mantra still holds true - practice, practice and more practice. I also try and get my clients to 'be in the moment' for five minutes every day at work. This means that they should try and be completely aware of their pronunciation for five minutes every day as a means to assist the transition from classroom work to every day speech.