Does personality change? Me and the Myers -Briggs

A few months ago Essie my supervisor had everyone in the department take a short online Myers-Briggs inventory and report the results back to her. Like a number of others in the department, I turned out to be an ISFJ (Introvert Sensing Feeling Judging). Essie remarked that everyone in the department was an introvert (except Able who, like Essie is an extrovert). It surprised me that I shared the same type ISFJ with two of my colleagues whom I think of as different types from myself. I've taken the Myers-Briggs a number of times and I usually end up INFJ (substitute Sensing for Intuitive) , though sometimes I've been an INTJ (substitute Thinking for Feeling) -- even once an INFP! (Substitute Perceiving for Judging). So what made the change from INFJ to ISFJ? , or rather from intuitive to sensing? Does this reflect the shift in library priorities from intuitive reference work to sensing programming ?
Years ago when I first took the Myers Briggs it was explained to me that, in a relationship over time, you develop qualities that compensate for your partners' deficits -- or qualities that make it easier to work with the person. Maybe that's right. Essie says she's taken the test at home and she's come out a different type from work. Another explanation for my variance is that I may have fairly even scores, which, for one reason or another tip one way depending on time and circumstance.
Is personality fixed and determined in childhood, or does one change? I believe to the extent of introversion-extroversion one stays the same throughout life, but it can vary in degree. Environment and life situation affect the other axes over time.There are things that one can do to change some aspects of personality, like become more comfortable with groups of people or learn to be more analytical, which could help also.
I expect that next time I take the Myers--Briggs I'll again turn out INFJ -- but won't be surprised if I vary one way or the other around the F/T or T/J axes. I seriously doubt that I'll end up as an extrovert!