My employer says the company was sold — do I have to sign a new contract?

No, not automatically. If the new owner has bought the shares of your employer, your employer is legally the same company as before — nothing about your existing contract changes just because the shares changed hands. If the new owner instead bought the business's assets and is taking you on as an employee of a new corporate entity, your service is still treated as continuous under the Employment Standards Act, 2000 for things like vacation and notice, and that continuity happens whether or not you sign anything new.

What a new contract is really for is changing your terms going forward — pay, duties, location, benefits. You are not obligated to sign a document just because it's put in front of you, and a new agreement only truly binds you if you get something of real value in exchange for accepting new terms, not simply the chance to keep the job you already had.

Before signing anything, ask for time to read it, compare it against your current terms, and understand exactly what's changing. If the new terms are worse, you have real options, and getting the document reviewed first is worth the short delay.