It's a pretty long explanation to get it to fit, but i'll try to make it brief.
you have to cut the entire backing of the grill that's attached to the bumper, so yes, the painted part where the grill sits. once you cut all those attachments off, you would have to bondo a new attachment piece to fit the jdm grill. since usdm and jdm bumpers have different tabs holding the grills up, plus the grill has a slightly different shape, so you would have to modify the entire backing in order to get it to fit.
just keep putting the grill on and off and while eyeing behind the grill keep making measurements and start layering up the bondo.
once everything settles, you'll have a new bracket to "sit" the new grill on. it'll take a good 4-6 hours depending on how good you are with bondo/cutting.