hi jonesy,
with my previous cooper,i retro fitted cooper s bumpers
to and i can tell you its very staight forward to do with no
problems whatsoever,the only thing to remember is if you
have the original side exit exhaust is you will have to swap
the black surround on the rear bumper with your existing
bumper.
also the front fogs need re positioning when you swap covers
this is very apparent how to do when the old cover is removed,
you just have to drill two small holes in the brackets that hold
the fogs to the car.
hope this helps.