How do you change or update a shareholder agreement in Ontario?

A shareholder agreement is a contract, and like any contract it can be amended by the agreement of the parties to it. Most shareholder agreements include an amendment clause specifying what level of consent is required to make changes — commonly unanimous consent of all shareholders, though sometimes a specified supermajority.

If the agreement requires unanimous consent for amendments, any single shareholder can effectively block a change they disagree with. This can be a feature or a bug depending on your perspective: it protects minority shareholders from having terms changed against their will, but it also means the agreement can become difficult to update as the business evolves.

Amendments should be made in writing and signed by all required parties. It's also good practice to review the agreement periodically — particularly when new shareholders join, when the business changes significantly, or when an existing shareholder's role changes. A new shareholder who was not a party to the original agreement generally needs to sign an adherence agreement (sometimes called a joinder) to become bound by its terms. A corporate lawyer can help you manage amendments and ensure the documentation is properly executed.