I wouldn't really worry about disparate impact here since there is a good and obvious business reason for exempting billable hour employees. It could, however, raise an eyebrow as to why it is that all the billable hour people are male, which would have more to do with hiring and recruitment practices than this particular decision althoug it would also make this particular decision look worse, too. hehehhehe
In any event, you can change pay going forward in the absence of any contract. Typical pay agreements stipulate that the comp plan can be changed. Typical handbooks say the rules are whatever the company says they are.