That's generally speaking, ill-advise, unless you know very well what you are doing in terms of heat-treatment. If you DON'T know what the heck you are doing with the heat-treatment, you'll end up weakening your frame to the point that it will be unusable. Brazing is only a small part of the story.

Finding a frame with horiz. dropouts may in the end be actually cheaper than having the vertical ones replaced.