I've just done this, but wondered how anyone else does it. My sequence was:

- turn the back plate up and hit the pointy-bits of the rivets with a short piece of round bar to flatten them slightly
- hit the newly flattened bits in the middle with a centre-punch, then put a 4mm drill into them until the rivet spins off
- using the same centre-punch, hit the rivets until they pop out of the back plate. You'll probably break the old HES but it doesn't matter at this point.

From here on in - mounting the new HES - was a bit of a puzzler. From playing with the OEM HES, I found that you could easily punch out the rivets without breaking anything. I don't have an anvil or a recessed punch of the right size (nor can I make them - no lathe), so hitting the new rivets and spreading them like the OEM ones was a no-go. The rivets on the new HES seemed to be moulded in, so they weren't going to punch out like the OEM ones. Cutting a long and sometimes frustrating story short, I did eventually resort to driving them out from the back with a hammer and punch and found that what appeared to be moulded plastic was actually epoxy resin. Bit of a heart-stopping moment though...

To actually fix the HES to the mounting plate, I took four 4mm button-head Allen screws and filed down the diameter of the heads until they dropped into the rivet recess in the HES. I also skimmed across the top of the button to make sure it was below the floor of the HES itself. I used Loctite threadlock on the butchered Allen screws and tightened the nut as far as I dared. I then hacksawed the threads flush with the nuts and ran a fine file over them to remove any sticky-up-and-cut-your-fingers bits. To add the belt to the braces, I centre-punched the nut to the screw. They is NOT coming off.

So... how do you guys refit the new HES?

I doubt anyone could improve on your method!  

on these newer versions with the built in extrusion of the base plate , likely the best idea is to drill them out , just makes sense and cuts down assembly time and complexity , 

good work Dai 

i chose to pin punch the new hes backing plate to expand to the mounting holes on the assembly last time i did it ,

It did occur to me much later (and obviously in light of the information gathered) that there might be an easier way to do it.

- break or lever off the OEM HES leaving the rivets intact
- punch out the rivets from the new HES (MUST be done from the underside as the rivets are a slightly different type to the OEM)
- check the new HES to see if they will just push-fit over the existing rivets (basically an interference fit); if not, gently open the holes up using a round needle file as a reamer until they do
- a dollop of epoxy in the rivet holes to replace the epoxy you pushed out when removing the rivets.

The only thing retaining the original HES in place is the wiring that folds through the hole and is secured to the mounting plate. The rivets simply ensure that it is in the correct place on the plate; they're not so much 'rivets' as just 'mounting posts'.