Lugs such as Nervex are made for fairly thick gauge mild steel plate that has been pressed and manipulated into shape and then welded up. Builders such as Peugeot did not thin down the lugs before or after brazing, therefore it is likely that the lugs still have plenty of meat on them..
If the builder knows what he or she is doing, they would not find it difficult to alter the angles of ther lugs by using a little persuasion and light blacksmithing..so to speak. It would be much more difficult, possibly ompossibe..but not quite if the lugs were investment cast...but what would be the point of altering investment cast ones in any case?
As for remelting the brazing alloy..the difference between the liquidus and the solidus range of most silver brazing alloys is not that great at all and can be very easily reached without any danger of burning the steel. Similarly joints brazed up with a copper/zinc/silicon alloy can be reheated to the point where the alloy is sufficiently liquid to enable the tubes to be extracted without any danger of spoiling the steel of the lugs.
The real danger arises from being tempted to use a too large and therefore extremely hot oxy-acetylene torch flame in order to ensure that the whole of the lug joint is heated to the appropriate temperature for melting the brazing alloy. Old fashioned firms such as Ellis-Briggs always dismantle frame joints when replacing tubes by using a large town gas-compressed air torch that gives a very large enveloping flame without the extreme heat of the oxy-acetylene...hot enough to do the job safely..it justs takes a little while longer.
If you look at the photos of Bill Hurlow put on to the List recently, you will see one shot of Bill brazing up what I think is the seat cluster/seatlug joint of a frame. In that shot he is using a compressed air/gas torch...and I have no doubt that he used the same torch for the bottom bracket and other main lugged joints. Bill started building in the 1930s and it is almost certain that he would have learned his trade using such a torch, as I did in the 50s. In fact I knew some builders who actually used a parafin blow -lamp - one of those devices that you have to pump up to increase the air content of the flame. Such heat sources are slower but nevertheless they are safer and lead to less likelihood of distortion and hot/cold breakages of tubes.
Norris Lockley
Settle UK