1) is an rcbo cu likely to suffer trips with 1972 vintage (ie rest of the house) wiring?
2) does installing a new cu require a pir to be carried out?
I'm imagining a snowball that goes directly from having just a very short section of rewiring into needing the whole house rewiring, at least in part to comply with 2014 regulations, with all the hassle and mess that brings with it.
1. Depends on the quality of the existing wiring. Unsatisfactory wiring should not be reconnected, but it doesn't have to be strictly compliant with latest standards to be acceptable for reconnection.
2. No, but it would be very unwise not to do an inspection before changing the CU, not least because it will make testing and certifying the new CU much much harder if there are uncorrected faults.
The most serious widespread defect likely to be found is no earth to lighting circuits. If that has been omitted they really should be recabled. Otherwise a 1970s installation if done correctly to the standards of the day and with good workmanship should be stubstantially reusable. The main issue is often later bodges to deal with the insufficient number of sockets provided originally. It is often possible to break 1 ring at a mid point, run 2 new cables back to the CU, and split the existing socket ring into two, adding new sockets as you go.
Electric shower if any will often be wired in 6mm cable for a 7.5kW unit and should be upgraded to 10mm for a 10kW unit. Immersion heater if used should ideally be moved off the socket ring onto its own circuit.