How far apart downlights are from each other depends on the size of the ceiling. To find the distance between them, you can find the height of the ceiling and divide that by 2. In general, do not exceed 6 feet apart between lights.
When installing downlights, it is also recommended to place them about 2 feet away from the wall.
There are a couple of ways to change to LED downlights. One way you can change to LED is by changing the downlight bulb.
Another way you can change to LED downlights is by replacing the entire housing.