Just because it is substandard does not mean you have to repipe it, you are
only "obliged" to do that if you are a professional, it would seem the
proposal would improve the piping capacity.
I had a telephone conversation with CORGI last year and who are aware the
advice they give out, if followed slavishly, would create havoc ie "gas
pipework undersized so that a gas appliance cannnot run at the
manufacturer's design input pressure (and flow)" = AT RISK
Well that would knock out around 80-90% of gas appliances.
The tech guy gave his opinion that cookers would be ok to 14mb, probably
the same for boilers, especially modern modulating, but he would not firmly
give this figure for boilers as there are too many variances.
The only real danger is cookers and some gas fires on a low peep can be
extinguished by the boiler kicking in, but this only happens well below
14mb.