The tray loading (233-333MHz) and slot loading (350-700MHz) iMac G3 logic boards are a completely different shape and size, with different electrical and cooling requirements, different monitor connections, so swapping from one family to the other isn't very practical. Some slot loading logic boards can be swapped into other slot loading iMac G3s, but there are some differences on later boards that may complicate things. I'm not as familiar with tray loading iMac G3s, but I believe the logic boards have interchangeability between revisions.