About changing Frame Rate Settings of a composition ?

If you are talking projects that do not contain video shot with a camera of real things you can change frame rates to anything you like at any time because keyframes are based on time not frames. The only "nudging" of keyframes to a specific frame that is ever required comes when you need some movement to stop or start on a specific frame because of things like frame blending.

The bigger question is what do you want the motion to look like. 29.97 is a video standard in the US, 25 is a PAL standard and things look more like TV at those frame rates. 59.94i is not usually 59.94 interlaced frames per second or approximately 120 fields per second, it is usually 29.97 interlaced frames per second where each frame is composed of two fields. 29.97P is exactly the same as 29.97I when you have identical fields - both fields are the same slice of time. The only visual difference between 29.97 interlaced and 29.97 progressive is that interlaced footage has fields that are different slices in time. Almost ALL broadcast over cable or the air is interlaced whether it is in an NTSC or a PAL country. There are very few channels that broadcast at high frame rates (59.97P) because the data rate is twice as high and bandwidth costs money. That's just how TV broadcast works.

The biggest difference between film rates 23.976, 24 and 25 and 29.97 or 30 fps is the potential problems you will have moving graphics horizontally or vertically. The higher the frame rate the smaller likelihood that you will run into critical panning speeds that cause the image to judder and become unwatchable. It is easier to animate at higher frame rates. Lower frame rates (24) produce more film like motion blur and a little more of a cinematic look, but you have to be much more careful with the speed of your camera moves and your animations.

Adding 3:2 pulldown is only necessary when you are converting 24 or 23.976 frame rate video to 29.97i (interlaced) because it only can be done with interlaced footage. Most folks make the mistake of thinking that 29.97i and 59.94i are different frame rates, they are not in most cases, it's just different terms for the exact same frame rate and the exact same number of fields. There are exceptions so you have to test your interlaced footage to see what you actually have.

So now are you completely confused? Make it easy on yourself. Unless your client has specific specifications for frame rate and interlacing and your client knows what they are actually talking about (many do not - and it's your job to know the difference) use the frame rate that gives you the look you want with the least amount of work. In almost every case anything above 29.97 will be played back at that frame rate anyway when you look at it on TV. 24 fps projects require more care in camera moves and animation speed than 29.97 (30) so if you choose to work at 24, or even 25, make sure you preview your animations full size at full speed to check for unacceptable motion artifacts because they are a lot more common than you think.