deanc;
Ah, the wonder of code interpretations. This is what makes my day (haha) in dealing with the NBIC main comittee. Anyway, the issue is "affecting the pressure retaining capability" because the example given in Appendix C-6 is a change in the original design of a heat exchanger head to a pressurized jacket design.
The information given by UKcats is sketchy in detail. If the replacement head is not changing the original design of the heater and all that is required is to drill larger bolt holes in the original head for a new, replacment shell (that is the same design as the old shell), I still believe this is a repair. All that has changed is larger bolt holes in the head - nothing else.
However, if the existing head is being placed on a new shell that is different in design - size, wall thickness etc (not an in-kind replacment of the original shell), this would be an alteration because the pressure boundary of the heater shell has been changed.