I have not been able to find a lot of information on frame repairs and alterations. There are some videos on the ubiquitous YouTube that cover the odd instance. The Paterek manual, one of the few frame building guides, does give some helpful advice and this advice certainly seems sensible. My take on this is that you should ideally AVOID THE TORCH. Obviously adding parts and remaking parts of a frame inevitably will involve heating and brazing, but this is best kept to a minimum on the basis that it will have least effect on the metal; the properties of which may be an unknown quantity. Also when brazing for a repair or alteration the use of silver, rather than brass would be the preferred option to keep the heating of metal to the minimum. Using a torch to melt silver solder may be acceptable as the melting point is relatively low, but the temperature needed to melt brass braze is higher than that needed to braze it in the first place.
Therefore where possible it seems sensible to remove existing frame parts by mechanical means, such as cutting, filing, grinding and reaming wherever possible to generate the least heat, before using the torch to add the new or replacement parts. I am of course talking about steel frames. Aluminium is difficult to repair and I suspect generally not worth it; and carbon fibre probably best left to a specialist.