I've done this exact conversion on my MTB that I used as a commuter for school. I made a jig to hold the two brake-bosses at the precise spacing. It was simply a 2x6" metal plate that I drilled two holes into and then bolted it to the brake-bosses. Then I used a Dremel cutting disc and trimmed the brake-bosses off as close to the tubing as possible. Moved the bosses up about 30mm and brazed them into their new location with an oxy/acetylene torche. Took about 2-3 hours total with clean-up and painting.
