Very much depends on action design, I'd think, as to whether a BPE is capable of being re-barreled or re-proofed as a Nitro Express.

I do know personally of one Double, but it was a 10 bore Westley Richards - practically new in case, action still a bit stiff, needing shooting to smooth it up. H&H said it hadn't fired a full box of ammo when sent to them for examination in the 70's, but my late friend Will Tompson & that the gun would "loosen up" with shooting. The higher regarded this gun, though made by a competitor.

H&H wrote Will asking him if he'd allow them to re-proof the gun for nitro as it was in new condition and a late gun as well & they knew the gun would pass proof, without any danger to the gun.

Will acquiesced to the request and received it back in fine shape, newly proofed and stamped for 2 7/8" NE shells - but then, that was a shogun, not a rifle.