My theory is that people are more inclined to switch between e/i and p/j than the two middle letters. In other words, people are willing to change the order and orientation of their top two functions, but they don't want new types of functions up there, because that would be too fundamental a shift. They're willing to change the style and shape, but not the content. So, for example:
ISTPs would think about switching to ESTP, ISTJ, and ESTJ, but not INTP, ISFP, or INFP.
ESFJs would think about switching to ESFP, ISFJ, and ISFP, but not ENFJ, ESTJ, or ENTJ.
INFPs would think about switching to ENFP, INFJ, or ENFJ, but not ISFP, INTP, or ISTP.
ENTPs would think about switching to INTP, ENTJ, or INTJ, but not ESTP, ENFP, or ESFP.
This theory jives with my experience; it might be cool to be more extroverted and/or more open ended, but I wouldn't want to be less abstract or less rational.