"Today, Canada and the United States reached an agreement, alongside Mexico, on a new, modernized trade agreement for the 21st Century: the United States-Mexico-Canada Agreement," US Trade Representative Robert Lighthizer and Canadian Foreign Affairs Minister Chrystia Freeland said in a joint statement. [...]
The joint US-Canada statement did not include details of the agreement. However, the new accord is expected to contain key provisions on Canada's dairy industry and car exports to the US.
So, what's substantially different in USMCA vs the old NAFTA?
This might not be answerable right now, but details are likely to emerge in the forthcoming days or even hours.
Actually some details already have emerged:
The officials highlighted in particular that the U.S. had won a “substantial” increase in access to the Canadian dairy market, and that Canada had agreed to end the “class-seven” milk program that undercut American sales of a special dried-milk product.
The same piece notes that chapter-19 dispute resolution mechanism was being kept, despite initial US demands to scrap it.