First off, you need a 1MB Fat Agnus in order to switch between modes on an Amiga. Assuming that's in order, you can flip between modes by soldering a switch between Jumper 102 or using a program such as Tude or Degrader after Workbench has loaded.
I remember bring able to choose between NTSC and PAL in the boot menu or was that only for the 1200/4000?
There are advantages and disadvantages between the two formats as far as Amiga gaming is concerned. You have the advantage as outlined by the poster where you have more realestate on the screen for graphics to go but PAL also runs a little slower due to the 50Hz vs 60Hz scanning frequency. Since PAL is(was) the standard for Australia, I know of a friend how did the mod to be able to switch between the two modes on his Amiga 500. He swears some games did receive a slight speed up.
At the time, I just remember being glad we had PAL so it meant we could play all the games from around the world
