All your really have to do is take in or let out at the center back seam. Tailors do this all the time. That narrows the chest which is really all the PTP measurement is. Granted, you don't want to turn a 46 into a 40, but if you're talking about going from a 40 to a 39, or even a 38, most tailors can do that. The absolute worst that could happen is he'd have to do a recut, but I don't think that would be an issue here. Recuts are expensive, but adjusting a center or side seem is affordable. Again, as long as the shoulders fit and the chest isn't enormous.
Edit: Different people will give you different answers to this question. That is true. But it is also true that this can be done. you can also take in a pinch at each side seem without affecting the armscye. All in all, if you take a bit at all the major seems, you can reduce the chest of a jacket without costing an arm and a leg.
Very interesting, thanks for the detail explication.