If we are revising our safe deposit contracts to include a right of offset provision, do we need to have existing SDB customers sign a new contract, or can we just give them 30 days' notice of the change in terms (similar to TIS disclosures)?

The answer depends on what your existing contract provides, if anything, with respect to its amendment. Did you reserve the right to amend the terms of the contract? If not, you may not be able to amend the contract. If your contract provides that amendments must be acceptable to both sides then you can't unilateral amend the contract. You may want to have your counsel review your existing contract to advise you of how and under what conditions you can amend your contract.