If an employee accepts a monetary value in exchange for signing a new employment contract which states prior service will not be recognized - would the date of signed contract hold up as enforceable in court to calculate termination pay?

The prior service is likely automatically recognized by operation of the employment standards legislation in your province and using a contract to exclude it would be ineffective. A contract is typically used to limit the employee's termination entitlements not to cancel their prior service but we need to know more about the situation you're describing and the province where you operate in order to give you a definitive answer and legal advice. I recommend you contact us at the number below to book a consultation to discuss the matter further.