Does an employee's probation period restart if the business changes hands?

Not automatically, and this is a distinction worth being precise about. A probationary period is generally a contractual and workplace-policy concept rather than a fixed statutory entitlement itself, so whether it "restarts" depends on what your own employment terms say, not on a fixed rule the way vacation continuity or notice entitlements work under the Employment Standards Act.

That said, an employee's underlying statutory entitlements — service-based vacation accrual, eligibility for statutory leaves, and how notice or severance would eventually be calculated — continue to run from their original start date with the seller if they're hired as part of a going-concern sale, regardless of anything your own internal probation policy says. You can choose to apply a new probationary period as an internal HR practice for onboarding purposes, but it doesn't undo the statutory service continuity sitting underneath it, and it shouldn't be presented to employees as though it does.

Be clear in any new documentation about what a probation period actually changes (internal onboarding and evaluation practices) versus what it can't change (statutory continuity of service). A Treadstone business lawyer can help make sure this distinction is reflected properly in your employment paperwork.