Can my old guarantee be replaced by my buyer's guarantee, or do landlords usually want both?

Either outcome is possible, and it comes down entirely to what the landlord is willing to agree to — there's no default rule that a new guarantee automatically replaces the old one, or that a landlord must choose one over both. Landlords generally see more security as better security, and unless you specifically negotiate a release, adding your buyer's guarantee alongside yours (rather than instead of it) is often the path of least resistance for the landlord, since it costs them nothing and gives them two parties to pursue instead of one.

Getting your guarantee fully replaced — meaning you're released and only the buyer's guarantee remains — requires an explicit agreement to that effect, ideally documented in the same consent package that approves the assignment. Simply having your buyer sign a guarantee doesn't, on its own, cancel yours.

Because this is easy to leave ambiguous and hard to fix after the fact, spell out clearly in the assignment documents whether your guarantee is being replaced or simply supplemented. A Treadstone business lawyer can make sure this is addressed explicitly rather than left open to interpretation.