Least changes = speakers -- just a size, space aspect.
Probably change amplifiers the most if anything, but its not necessarily an upgrade thing more than a curiosity thing. It may be better, it may be not, but my system changes tend to not always be a linear, 'better' sort of path, but a want to try difference perspectives, even with known shortcomings.
For example, if I loved a manufacturer's middle of the road amplifier or speaker, and knew that there was another step up, there would be a part of me that would want to try 'better', and there's a part of me that trying something totally different, from a totally different philosophy is just as interesting to do....