No. Ontario's Labour Relations Act, 1995 provides that once a trade union applies for certification at the Ontario Labour Relations Board and the employer has notice of it from the Board, the Act establishes a "statutory freeze" until the Board has issued a final decision in the application or the union has served notice to bargain (ss. 86(2)). Similarly, if the Union wins certification, it has the right to serve the employer with a notice to bargain toward a collective agreement. That notice to bargain triggers another statutory freeze, effective until an agreement is negotiated or the union is in a strike position (ss. 86(1)). Under both 86(1) and (2), the freeze is broadly worded and applies to wages and "any other term or condition of employment or any right, privilege or duty of the employer, the trade union or the employees". Benefits would definitely be frozen, unless the Union consented to a change.
Similarly, since the Act requires "business as usual" during the freeze, an employer would have to follow its normal practices (e.g., annual evaluations and wage increases) and implement any planned improvements or increases in working conditions in order to avoid violating the statutory freeze.
During an organizing campaign (that is, before the freeze kicks in), the removal or threat to remove, benefit coverage would be an unfair labour practice complaint if it is motivated by anti-union animus, seeking to interfere with the formation or selection of a trade union. Employer interference in trade unions and employer interference with employees' rights are outlawed by sections 70 and 72.