Can they demote me without warning right after the sale goes through?

Not without real consequences to the employer, even though a new owner does have some legitimate room to restructure. If your employment continues after the sale, your job — including your position, responsibilities, and status — is protected by the same basic principles that applied before: an employer generally can't unilaterally impose a significant demotion, whether in title, duties, pay, or reporting relationship, without it amounting to a fundamental change to your employment.

A new owner reorganizing how the business runs can make some adjustments, but a serious, unannounced demotion imposed right after closing can be treated as ending your existing employment in substance, even if you're still nominally on the payroll — which can trigger rights beyond the statutory minimums under the common law. Continuity of employment under the Employment Standards Act, 2000 is exactly what preserves your standing to argue this; your history with the business didn't just start over because ownership changed.

If you're demoted suddenly and without warning or real justification, don't assume you have to just accept the new role. Document what changed and when, and get advice promptly, since acting quickly preserves your options.