CX should be updated to fetch data-mw in a separate API call from RESTBase. Whether CX wants to inline the data-mw into the HTML or handle data-mw separately would be something for CX to resolve. But, processing the data-mw by inlining it into the HTML would be a straightforward way to handle this change. Parsoid (and RESTBase) will accept inlined data-mw or a separate data-mw blob in the html -> wt API end points.
In order to minimize the performance impact in the Parsoid production cluster from having to return the old version, RESTBase and Parsoid won't bump the HTML version number in production till CX and other Parsoid HTML clients are ready to consume this newer version. So, it is good to start work on this sooner than later.
At the time of deploying this change to production, CX would also have to update the version string in the Accept: header based on the updates T130638: Add data-mw as a separate JSON blob in the pagebundle output of Parsoid's API.