Legal Problem or Not re: Unfair Treatment?

We have a Project Manager career track that has the following titles:

Assistant Project Manager
Associate Project Manager
Project Manager
Sr Project Manager

Each title represents a higher level and salary. At the Project Mgr & Sr Project Mgr level, employees can either be in an individual contributor role or in a supervisory role. Several different depts have the individual contributor career track but not all of those departments have the supervisory career track.

In the past, the only way to move up a level was through a promotion. All promotions have been frozen so now, we are thinking about allowing employees to move up a level only in the supervisory career track thru our job posting system where before they could only tranfer over laterally thru the job posting system.

We are also thinking about changing the qualifications from X number of yrs of relevant exp to X number of yrs of service with the organization (e.g. you would have to been with us anywhere from 2 to 6 yrs before being able to move up a level in the supervisory career track).

The depts that don't have the supervisory career track are upset and claiming unfair treatment since employees in those depts can't move up a level in their own dept and don't feel they would be selected for a move up in another dept since they don't have supervisory experience nor specific experience working in that different dept. They feel the employees in those depts will receive opportunities to move up a level and they won't because of this which is likely true since those employees do have a competive advatage of hands-on experience working in that particular department.

If we decide to make this change, are there any problems from a legal perspective?

Would the employees from the depts that don't have the supervisory career track have a strong case if they were to sue us?

Thank you for your assistance and looking foward to your replies.