You can pay the $25 and complete Profile before any final decisions have been made on which schools the student will submit an application to. The completed Profile will just be sitting there waiting for you to log in to your College Board account and pay $16 to send it to each school that will receive an application and requires the form to be considered for need-based aid.
To be clear, you don’t amend the Profile - you simply instruct the College Scholarship Service to send it to certain schools, as required, when a decision has been made to apply and before each school’s deadline for submission. You can do this for as many schools as you want (or until you run out of money to pay $16 for each submission), but it will be basically the same Profile sent to each school.
No, you don’t amend FAFSA over and over (unless you keep making mistakes in entering information). What you can do, as previously explained, is modify the list of schools where FAFSA will be sent. The financial and personal information should not be changed, assuming everything was done correctly.