I've been Instructing for the past 12 years.
My students always have the right to change instructors and I let them know of that at the beginning. It's their right.
HOWEVER, I would be very reluctant in being someone's 3rd or 4th instructor during their PPL license. Let's face it, if you have to go thru handful of instructors, the problem isn't the instructors. Yes there are exceptions to this such as instructor leaving the school or simply a poor match between the CFI and student.
When we have a student in our school on their 3rd/4th instructor for a SINGLE license, some of us CFI simply ask "what's the problem with the student?"