Do I have to reapply for my own job just because the business was sold?

Not because the business changed hands — but the answer depends on which kind of sale happened. If the new owner bought shares in your employer, your employer is legally unchanged, so there's no "job" to reapply for; you're already employed, full stop. If the new owner instead bought the business's assets, they are legally a new employer, and technically that new employer chooses who it hires — it has no automatic obligation to take on any of the seller's staff.

In practice, most buyers of an ongoing, operating business want continuity and simply keep the existing team on, often through a formal offer letter rather than a real reapplication process, and Ontario law treats your service as carried over for entitlement purposes when that happens. Some buyers do run a more formal process anyway, for optics or due-diligence reasons, even when they intend to keep everyone — that's not the same as your old job legally disappearing.

If you're asked to "reapply," ask directly whether this is a formality or whether the buyer is genuinely deciding who to keep, since that changes how seriously to take the process and what to ask for in return.