The difference is that the new design no longer has that little "paddle" that fits under the right grip to keep it in place.
Its pretty easy to fit a safety but if you don't know how or aren't "handy" you can have a smith do it. It shouldn't cost over 25 bucks. Since they are MIM, there is a good chance that it won't require fitting and will just drop in.
If you do a search on fitting a thumb safety, you can find posts with instructions on how to do it.
The difference between the ambi safeties is that the new ones
include a dovetail hammer pin that fits into a dovetail slot on the weak side of the safety. This holds the two pieces of the safety together much better than the old style that relies on the grip panel to keep it in place. King's Gun works in California(excellent 1911 products) has been using this design for years. I wonder if Kimber had to get permission to use King's design? I just installed the new Kimber style last week. Here's how I did it:
I used a black sharpie pen to color the entire panel(the part of the safety that rides against the frame) before installing it. I installed the safety. Before insalling the grip safety or Mainspring Housing, I worked the safety on & off A bunch of times. I removed the safety and looked for places where the marker ink had wore off and for wear marks on the frame. I first used a very small, fine flat file then used emery cloth then used 400 grit sandcloth to smooth out the places where I saw wear marks. I wiped of the safety, colored it again with the sharpie & repeated the above steps until it felt right. Once I got it wear I wanted it. I used 409 household cleaner then CLP to completely remove the magic marker ink. This ink will mix with certain gun oils and make parts bind & stick - I had it happen once! then I wiped the part dry and reassembled the pistol. It took me about 45 min to fit this part. But that's the joy of the 1911. It's a tinkers gun.