I did it today. It really sucked changing the lower mesh. I did it without taking the bumper off. The first thing I did was cut the mesh off the frame of the grille. Then took a pair of wire cutters and a box cutter to cut around where each clip snaps in. Once I cut around all of the clips, I cut the grill into probably 6 pieces. The grille needs to be pushed in to undo the tabs and then pulled out when you get them free. You really need to look at the new grille your putting in to see how it goes in, its really weird. At first it didn't look like the new one would fit. Just squeeze it in there, bottom first then the top, and then pull the grille backwards for it too snap in. I had to pull on the plastic of the bumper to get it to fit. Job took about 45 minutes. Really not fun, but worth the result.
Here are some pics
http://www.xbimmers.com/forums/showthread.php?t=525141