Generally a company cannot terminate its plan and start a new one to avoid some employees obtaining the vested benefit their service would entitled them. In most circumstances any prior service would be computed on the new plan's vesting schedule.
However, there are exceptions to that general rule based on why a new plan was created, generally related to a change in the employer's business, such as a bankruptcy, merger, etc. If the company is denying you appropriate vesting you should file a claim on the plan to get them to provide you with a written explanation. The process for filing a claim will be listed in the SPD.