You offer no facts at all regarding illegal discrimination. Discrimination and treating people differently is not illegal. The only unlawful discrimination is that which is based on race, religion, ethnicity, age, gender, sexual orientation, disability, and familial status. But you have no evidence. A court will not entertain hearsay, conjecture and speculation. Even where one of these protected categories apply, you'd have to first file a claim with the EEOC and they'll give you paperwork permitting you to sue depending on what their investigation reveals. But you have offered no facts or evidence that any of this is related to your status in one of these protected categories.
The business if free to promote and demote people as it sees fit, and they have no legal obligation to discuss it with you. Replacing you with someone of the boss' own race does not prove racial discrimination. If the boss can articulate any other reason that this person was given the position over you, that is usually sufficient legally. Keep in mind they did not cut your pay.
Keep in mind that TN is an "at will" employment state. This means you can be fired at any time without just cause as long as it is not related to your race, religion, ethnicity, age, gender, sexual orientation, disability, familial status, and not in retaliation for whistleblowing illegal conduct, taking FMLA leave, and filing for disability/workers compensation.
If you have more than what you've presented, use the "find a lawyer" tool on AVVO to get a consultation with a local plaintiff employment lawyer. Many give free consultations, check beforehand.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.
You mention the possible promotion of a "younger person" into your job. It is possible that you have a case of discrimination based on age. The Age Discrimination in Employment Act (ADEA) forbids age discrimination against people who are age 40 or older. This depends on what evidence you have that she was promoted for being younger as opposed to the mere coincidence that she is younger.