A loan is basically a contract; neither party may unilaterally, or on its own, change its terms. That applies also to a new lender, who buys or otherwise acquires the loan from the old lender: it is still bound by the same terms and conditions as the original lender. So if the original terms and conditions of the loan would allow the interest rate to be changed now or under these conditions, then it can be changed, but not otherwise. You need to refer to the loan documents to see what the lender, whether old or new, can and cannot do.