Can I lose my right to claim constructive dismissal if I keep working after the change in Ontario?

Yes. If you continue working for too long after a significant unilateral change without objecting, a court can find that you accepted, or condoned, the new terms, which can undermine a later constructive dismissal claim based on that same change. Courts look at how much time passed, whether you raised objections or expressed disagreement along the way, and whether continuing to work looked like genuine acceptance of the new arrangement rather than a reasonable, temporary decision to keep earning income while assessing your options.

There is no fixed cutoff for how long is too long - it depends on the nature of the change and how you responded to it in the meantime. Employees who want to preserve a constructive dismissal claim while continuing to work are generally well advised to clearly communicate, ideally in writing, that they consider the change unacceptable and are continuing to work under protest or while evaluating their options, rather than staying silent and risking the argument that their continued work amounted to acceptance of the new terms.