Inside US Trade has a copy of the Trump administration's draft notice to Congress "that the President intends to initiate negotiations related to the North American Free Trade Agreement (NAFTA) and its architecture," in which they set out "specific objectives for negotiation." Not surprisingly, the administration wants to eliminate NAFTA Chapter 19:
Eliminate Chapter 19 dispute settlement of antidumping and countervailing duty determinations in light of U.S. experiences where panels have ignored the appropriate standard of review and applicable law, and where aberrant panel decisions have not been effectively reviewed and corrected.
More broadly, they seem to want to make NAFTA more like the TPP. Here are some examples:
Seek to have the NAFTA countries improve upon their WTO TBT commitments and eliminate any unjustified TBT measures.
Maintain and seek to improve procedures to resolve disputes between U.S. investors and the NAFTA countries through, among other things, mechanisms to deter the filing of and eliminate frivolous claims; procedures to ensure the efficient selection of arbitrators and the expeditious disposition of claims; and procedures to ensure transparency and public participation in dispute settlement proceedings.
Seek commitments from the NAFTA countries not to impose customs duties on digital products or unjustifiably discriminate among products delivered electronically.
Consistent with U.S. priorities and objectives, seek a commitment by the NAFTA countries to adopt and maintain measures implementing internationally recognized labor rights and effectively enforce their respective labor laws concerning those rights.
Note the second item there: Sounds like ISDS is staying in.
There are also some new, potentially big, items in there, relating to U.S. complaints about border adjustment taxes imposed by other countries, Buy America procurement, rules of origin, and "snapback" tariffs:
Seek to level the playing field on tax treatment.
Seek to establish rules that require government procurement to be conducted in a manner that is consistent with U.S. law and the Administration's policy on domestic procurement preferences.
Seek rules of origin that ensure that the Agreement supports production and jobs in the United States, procedures for applying these rules, and provisions to address circumvention that ensure that preferential duty rates under the agreement apply only to goods eligible to receive such treatment, without creating unnecessary obstacles to trade.
Seek a safeguard mechanism to allow a temporary revocation of tariff preferences, if increased imports from NAFTA countries are a substantial cause of serious injury or threat of serious injury to the domestic industry.
It's very unclear where the NAFTA renegotiation will end up, but at first glance I would say this is more than a tweak, but far from blowing up the system.