Its been a while, but I think employment contracts fall under the Statute of Frauds and have to be in writing to be enforceable. Maybe a Tennessee contract attorney can help.
While I am by no means a contract attorney, I am an attorney (and this does not constitute legal advice). That being said, in other types of contracts, that is not necessarily the rule, even though it is the rule (I know that doesn't make sense, but honestly nothing really makes sense in the law). Example, a few years ago, a Tenn. Court of Appeals case, McKinnis v. Hammons, said that even without a written contract in a real estate transaction, the court could look to checks written by the parties and MLS listings showing a property was being sold, and signing of deeds by both parties as evidence that a contract existed, even though no sale contract was ever written. Now, no good real estate agent would ever not have a contract, but private parties who are less knowledgeable do things on the fly all the time. This effectively circumvented the Statute of Frauds in that particular case, and to my knowledge and according to Westlaw, it hasn't been overturned, but it caused quite the "wait a minute" moment in certain circles when it went down.
That being said, and having worked in firm that did employment law, employment contracts are a separate beast all together especially because Tennessee is an at will employment state. If there is no contract, an employer can fire you for whatever reason they want to give, and you can leave for whatever reason you want to (the Cliff's notes version). So if you want to outline specifically the expectations, obligations of the parties, requirements of the parties, and what happens if $h!t goes sideways, you need and want it specified in writing. And generally, verbal employment contract is a no no. Modifying an existing contract verbally is a no no. You always want to have everything written down and detailed and signed off as agreed upon. But there are always exceptions in law so if it can be documented enough that a "mutual understanding" was reached without a contract, it is always possible, however unlikely, that unwritten crap can get introduced and enforced.
And that ladies and gentlemen is the most law I have practiced this morning because of the rabbit hole that is this thread.
*caveat - I see I'm like 10 pages behind already...curse you thread...curse you.