Can a union grieve the sale itself, separate from grieving anything I actually do afterward?

It depends on what the applicable collective agreement actually says, since this isn't a general rule that applies the same way to every unionized workplace. Some collective agreements include specific provisions addressing what happens on a sale of the business — requirements for notice to the union, successor obligations, or conditions the parties negotiated in advance for exactly this scenario — and a failure to follow those specific provisions can itself be grieved as a breach of the agreement, separate from anything about how you subsequently treat employees.

Where a collective agreement doesn't specifically address a sale, a grievance is more likely to focus on the practical effects of the transaction as they play out — how employees were actually treated, whether the agreement's terms are being honoured going forward — rather than the mere fact of the sale itself, since simply selling a business isn't automatically a violation of a typical collective agreement.

Review the specific collective agreement's provisions on business sales and successor obligations before closing, since they may impose notice or process requirements independent of anything you do afterward. A Treadstone business lawyer can help you identify and comply with these obligations.