Do a commercial tenant's lease guarantors remain bound after the building is sold to a new owner?

Generally, yes. A lease guarantee is a separate agreement in which the guarantor promises to answer for the tenant's obligations under the lease, and because standard commercial lease language typically allows the landlord's interest, including the benefit of any guarantee, to be assigned along with the reversion, guarantors usually remain bound to whoever becomes the new landlord after a sale, without needing to sign anything fresh.

The nuance worth checking is that this depends on the actual wording of both the lease and the guarantee agreement, not just a general assumption. Some guarantees are drafted narrowly to benefit only the original named landlord, or contain conditions or notice requirements tied to a change in ownership; others are drafted broadly enough to follow the lease automatically. A buyer relying on an existing guarantor as part of underwriting a deal should have their lawyer specifically confirm that the guarantee is assignable and will remain enforceable against the new owner after closing.

Because a guarantee that turns out to be unenforceable against the new owner can materially change the risk profile of a tenancy, this is a detail worth verifying during due diligence rather than assuming it will simply carry over.