It's certainly very difficult to change motor memory, which is what handwriting becomes. I don't think it's so much that it can't be done as that it can't be done trivially - it may require intensive practice (which may be resisted) and supervision (which may be hard to come by).
The great thing about the grips I linked to above is that they prevent the child from wrapping the thumb - the crossover grip by having wings in the way and the jumbo is simply too big to get all the way around. Thus supervision requirements / constant corrections are reduced.
