Each state has their own State Insurance commission/department. They set policy and state legislatures make state law that governs the rules insurance companies must follow to offer insurance in their state. Guess what, states determine what rates insurance companies can raise and when. Example: States often allows insurance companies to raise rates a maximum of XX% after a certain set of triggers are met. Be it auto/homeowners, etc. So when you move from one state to the next, you fall under new laws that USAA/State Farm etc must follow. The individual company chooses how to apply the min/max of the laws. i.e. Insurance companies may make it policy to charge max allowed by law. USAA in my experience would raise rates the max allowed by my current state law. When you move, that is the determing factor. Not your old state laws. Common sense for USAA would be to look at old rate, look at new state law and try to keep you close or lower to keep business. Maybe at one time they followed that business policy since we are a member company (not a pure for profit company) More like a credit union concept. Any way, MS allowed and controls what insurance companies can charge and offer in MS. USAA then works between the lines. Hope this helps. Here is the MS link for your situation, check the laws/policies section.
Mississippi Insurance Department