Yes, you can change the college branch after 1st year.
Branch up-gradation is the opportunity given by the college, not by PMSSS, PMSSS allot branch at the time of choice filling.
If your college is allowing you to upgrade branches and know you are studying under PMSSS, it is possible to upgrade the branch. In that case, your scholarship will still come from your previous branch and will not stop and you will still get the free education.
But in some colleges students from PMSSS are kept in the different quota of seats and branch up-gradation is only for those students who are not from PMSSS and paying their fees.
Suppose there are only 60 seats under one branch say it x and there are a total of 65 students under that branch 60 normal seats, 2 from PMSSS, and 3 students from Migration. Now it is up to the college that whom they allow for branch up-gradation usually it is for those 60 students but if college will allow then you can upgrade your branch and this will also not affect your scholarship.
I don't know whether PMSSS upgrades your branch in your profile or there is any other mechanism where the college may help to edit your PMSSS portal for the branch. Maybe some other students may know about it.