Although the federal Section 7(i) exemption is out there, I am not so sure it would apply to this person's position unless it was a retail establishment, which has not been clarified. Unfortunately I know way too much about 7(i).
There are two different things that 7(i) has to calculate if the exemption applies:
(1) the person must be paid at least 1 1/2 times minimum wage for every hour worked
(2) during a representative period, over 1/2 of total wages must be from commissions.
Failure on either of those points will cause the federal exemption to be null and void (on the second one, it is null for the next representative period).
As to how the contract reads and whether it can be changed retroactively, that is dependent on the details. I would suggest taking it to a local attorney.

