As OWD rightly says, this does not alter the barrels state of proof.
You would have to do something to the barrels or action that arguably could alter their pressure handling characteristics (PMC).
Examples of this would be lengthening the chambers or forcing cones, lapping the barrels to greater than their nominal proof size bracket, conversion to ejector (although in many cases this does nothing to the stength of the gun).
Improving the jointing only 'improves' the PMC, or so one could argue.
Interestingly, contrary to popular belief, there is no real minimum wall thickness criteria in the proof test, only a recommendation that they should be in excess of 0.020". I have had guns reproved down to MWT's of 0.014" (not recommended but the owner insisted) and the proof house never turned a hair. I am quite cirtain that they never check MWT's, just expect people to stick to their recommendation.
This raises the spector of freshly re-proofed guns with unacceptable (to most) thin walls and I would recommend all purchasers of old guns to insist on an assurance of wall thickness from the vendor, especially buying at auction.
An auction house recently offered a freshly reproofed hammergun by an illustrious maker with MWT's of around 0.014" and didn't publish the MWT's. It was alleged that a prospective purchaser enquired about it to be told of 'acceptable MWT's' but no figures.
