I actually think it's not really relevant that it may have met the regs in 1966. There weren't as many PEN faults then!
Isn't the wider principle that if you are significantly altering an installation, the right thing to do is to ensure the alteration complies with the regs today. BPG1 says this on page 5 before proceeding to give questionable advice.
At the end of the day you are putting your name to an EIC saying it complies with BS7671:2022 (unless you specifically list this as a departure of course, and then it's supposed to offer an equivalent level of protection, so I don't see how that stands up to scrutiny)
Personally, I think the "it will probably be ok" get-out-clause in BPG1 is daft and is exceptionally daft if it's PME, as the point is that with no warning a PEN fault could see the bonding carrying the entire return neutral current. There wouldn't be any signs of thermal damage if it hasn't happened before (as
@Aaron b said)
Regarding "ripping a house apart", the regs do allow bonding to be "as close as practicable to the point of entry", and on a couple of occasions I've left original undersized bonding in place and added some new 10 sq mm to the nearest convenient point.