MI contract law, can a primary borrower amend a loan with consent of the co-signor

Generally neither the borrow or the coborrower can do anything by themselves unless either (1) the financier (bank, loan company, lender, etc) lets them do it and the other person is let off the "hook" in the process, or (2) something in the loan contract paperwork actually gives the borrower the right to do that (and I've never seen it); otherwise they both have to go along with the change and the financier has to agree to it too. However the law may be different on this point in your state so you should check with a local attorney.