In the UK, lengthening the forcing cones does technically take the gun out of proof. This was established by the Proof Houses back in 90's when doing so was all the rage.
However, since there is no standard forcing cone, I can't see how the Proof House could establish the forcing cones had been lengthened (unless they were told by the perpetuator) and so declare the gun out of proof.
Very old guns had very abrupt forcing cones so a nice gentle FC in an old gun that had not been reproofed in the last 50 years could be assumed to have had the FC extended but it would be impossible to prove.