HR zone change during race?

Possibly, but not necessarily.

Nope.  Dangerous ground there. 

OK, thanks.  Can you tell me why it is dangerous?  (I'm not being snarky.  I want to understand.) Is it just because there is so much variability from one person to another, or do you have something else in mind?

not JK but I think it is because each athlete is different and HR is already variable hence differences can vary significantly. Some athletes might not exhibit as much variability while others do.

In my experience my HR tends to vary as much as 5 bpm from day to day and as much as 10bpm on race day hence *I* decided to ditch it and instead train/race by pace/RPE for swim/run and power/RPE for the bike. Although if I didn't have a power meter I would be using the HR/RPE for the bike for sure.

Yeah, like Jorge said and you guessed, variability beween individuals.  Even for the same person, the change on race day can vary up, down or not at all.  "Dangerous" in the sense that if you go into a race expecting to allow your HR to go higher you may end up doing a lot of walking by the end.  The longer the event (and the less experience you have in racing it and gauging RPE), I generally think it's better to stay conservative with any "zones".  In shorter races, you're likely to pretty much ignore HR anyway.  Those can be good opportunities to develop a "feel" for racing effort.