It appears your idiocy is preventing you from understanding what "contract" means under the law.
The rent amount will not be changed by the NRST because the NRST cannot arbitrarily change the terms of any pre-existing contract.
Nobody but you is saying the rent amount will be changed by the nrst.
What is being said is that after implementing the nrst, landlords will change the rent in their contracts to an amount that maximizes their profits.