I have insurance with restricted mileage. At the time they asked me the mileage, my odometer was broken. Since they didn't ask anything about the truck's condition, I answered their question truthfully.
Since then, I've replaced the odometer with another unit that works just fine. I haven't yet come to my renewal date, but I doubt they will ask any questions, especially once they have the check in hand.
There is no law that I'm aware of in my state that requires me to notify my insurance company if I modify the odometer. The only law I'm aware of regarding odometers is during titling, and that requires me to state whether the odometer reading is accurate. It hasn't been accurate since it turned over 99,999 miles.
You wrote that it is stuck on 310000. Unless you've changed odometers, I bet it doesn't clock that high. If yours is like mine, it turns over at 99,999 going back to 00000.0
My recommendation is to answer their question truthfully and leave it at that.
Bill
