I would think that re-heating the brazed joints should work well enough to release the frame tubes.
But, in this instance of the Peugeot lugs...
Who would have a spare Metric diameter tube set?
... uh... and a Metric bottom bracket shell?
Also... I'm not a frame builder, so I wonder how closely the frame geometry of a re-built frameset would need to resemble that of the original donor bike.
As I recall, there were quite a few different lugs offered in that very same lug pattern, in order to achieve different frame angles for a built up frameset.
So, another question might be: Would the frame geometry range allowed by the particular Nervex lugs which were used for that Peugeot racing bike also be acceptable for one's own intended transplant?
I fear it's all a bit more complicated than having a plumber re-solder 90 degree elbow joints onto new water pipes to relocate a sink or toilet.
BOB HANSON, ALBUQUERQUE, NEW MEXICO, USA