Originally Posted by Salamandrine

Doug Fattic is correct. To align a frame properly a well equipped framebuilder with a surface table or equivalent and other special tools is required.

However, a well equipped bike shop that knows what they are doing is good enough for most purposes for realigning your rear stays a couple mm. If, for example, your seat tube and head tube are already cattywampus, they aren't going to get any better, but in general the resulting alignment should be as good as before. It is important to realign the dropouts and the hanger. Misaligned dropouts can potentially cause broken axles, broken dropouts, and broken chainstays. Misaligned hangers mean the rear derailleur will never shift as well as it could.

Anyhow, if a bike is really super expensive, and/or the bike is not tracking well or doesn't ride no hands, it may be worth taking to a framebuilder. For a simple 126 to 130 conversion, IMHO a good bike shop with the right tools can do it well enough. The main problem may be that a good bike shop with people that have steel frame experience has become increasingly rare.