People can move their bitcoins between now and when the hard fork occurs. Creating a hard fork in the past will invalidate all the blocks between then and now, and cause a major disruption. It will never happen.
Most coins < 10 BTC won't be moved until the hard fork date. And if someone moves the coins to addresses with balances < 10 BTC they will be valid.
You are proposing "freezing" all addresses with more than 10 BTC. I'm saying that won't work because people with addresses holding more than 10 BTC will just spread them out to more addresses, and nothing will change other than now there is an artificial limit on the number bitcoins at an address.
You wrote that they couldn't move their bitcoins because the hard fork would occur in the past. I'm saying that doing a hard fork in the past is not feasible.