Can an insurance agency change your policy and raise your premiums for no valid reason?
They cannot change the policy or raise your premiums during the current term of insurance (e.g. if it's a one-year policy, during that year), but may do so upon expiration of the current policy and for its renewal. During the term or period of time of a policy, they are locked in contractually to the policy's provisions and premium, but once the policy expires, are free to make alterations so long as they provide you notice of the changes (e.g. a statement of the new charges; a new policy or policy addendum containing the changes).
They don't need a reason to change the policy or premiums: your recourse, if you don't like the changes, is to go and get insurance more to your liking, from a different provider.