CHC exchanges can occur. But it is not as simple as finding a student to switch with (although that is the first step). The RDMEs (Regional Director of Medical Education) from each site must also approve of the switch and it has to go through some of the higher-ups in admin at the Mesa campus. It is possible to make a switch, although not easy.
And yes, the 10 spots for each CHC are set...this is because there may be other students from other schools at the same clinic/hospital as you so they cannot accommodate an extra student. Or the clinic does not have the capacity for extra students. It takes a lot for a physician to take on a student as a 2nd year. There is a lot of extra time, teaching and commitment they put in to allow you access to patients at such an early stage in your medical career.
Some spots also open up throughout the application cycle as students get accepted to other school. We had students coming and going up the first day of classes. Students may also drop out during the year (this is rare but does happen) or may leave after 1st year (again, rare). So spots can also open up that way at various CHCs.