We currently have a kInvalid which doesn't seem to be used anymore. It was added in SERVER-50706 (and then lots of refactors happened to move it elsewhere). We used to have FCV constants that were more generic like kUpgradingFromLastLTSToLastContinuous, and we'd use kInvalid when lastLTS was equal to lastContinuous since it shouldn't be a valid transition. Since that isn't relevant anymore, and kInvalid isn't used anywhere in the codebase we should investigate if we can remove it.