I've been doing the same role as an admin assistant to a VP for 6 years. As of last month, I have a new VP I support. I expressed concern to her that my peers are all being promoted to Executive Assistant and that I thought I was in the wrong job classification due to her ranking. She later confirmed that my suspicions were correct and that they would work on reclassifying me in the coming months ahead. Pending the old VP had a higher rank than the new VP, am I owed retroactive pay for being in the wrong job code for six years? At our company, the title and pay is based upon the level and ranking of the executive you support and I am a hourly paid employee. Thank you!
Read your contract and employee manual.
If you can easily prove you're owed money based on those documents and your pay stubs, call the Department of Labor in your state for advice. https://www2.illinois.gov/idol/Pages/default.aspx
They take care of "stolen wages", but I have no idea if this counts as a "stolen wage". Also, note that there is usually a statute of limitations. I don't know how long it is in Illinois.
Be sure to put everything in writing. Even if a discussion happens verbally, memorialize what was said by email afterward. Then print out those email threads and keep those printed out copies at home.
Do not threaten your employer with going to the Department of Labor. Call your Department of Labor first to know what your rights are on this issue.
Of course, you could also try consulting a lawyer, but that might get expensive. How much money are we talking about here?
Also, if worse comes to worst, note that Small Claims court in your state could be an option. It's just that your claim would have to be limited to no more than $10,000 (plus court costs). https://ag.state.il.us/consumers/smlclaims.html
But the good news is that the statute of limitation for "written contracts" in Illinois Small Claims Court system seems to be 10 years.