Can a seller update the disclosure schedules right before closing without the buyer's agreement?

Not unilaterally, in the sense of having free rein — whether and how a seller can update the schedules before closing is governed entirely by what the purchase agreement itself says about schedule updates, and this is a heavily negotiated mechanism precisely because it affects who bears the risk of things that change or come to light between signing and closing.

Many agreements do give the seller a right, or even an obligation, to deliver updated schedules shortly before closing reflecting new information — but a well-drafted update mechanism typically comes paired with buyer protections, such as a right to walk away, or a right to still claim indemnity for a problem an update reveals, rather than letting an update simply cure the issue and eliminate the buyer's recourse. Without those protections, an unrestricted seller update right can effectively let the seller disclose its way out of a breach on the eve of closing.

If your agreement is silent on this, or gives the seller broad update rights without corresponding buyer protection, that's a gap worth flagging and negotiating before signing, not something to discover the value of only after a late update actually happens.