Does a SGT get discharged from the Army if he transitions to becoming a warrant but later reverts back to his/her SGT rank after 14 yea RCP?

Although it's not routinely done, there could be a number of reasons that a Warrant Officer would revert to their prior enlisted rank, but the most common reason would be so they could continue on active duty and receive a regular retirement.

Specific to your question, there isn't an exception to RCP for an officer or warrant officer reverting to their enlisted rank (if held) prior to becoming an officer.

In the hypothetical situation you gave (Warrant Officer reverting to SGT and have over 14 years active duty service), and assuming you were eligible (AR 601-280, para 1-8d)*, you would have to obtain an ETP waiver in order to reenlist if you're past your RCP.
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* AR 601-280 (Army Retention Program) - https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN35366-AR_601-280-000-WEB-1.pdf

Seen it happen. Associate was a Flight WO1, then his hearing tripped him up medically, then transitioned to intel MOS, and tried to apply for Intel WO from SSG, but was told too bad so said, you failed out of the flight WO program so that was your WO chance.

Totally blows.............

In A Similar Situation, There Was A WO-1 Who Was The Maintenance Officer In My Last Company , Who Reverted Back To SSG ,
Because He Could Not Get Promoted To WO-2 For Failing A Required School .