This kind of change usually brings its own set of challenges because it is in a fundamental aspect of the UI. Typography, spacing, coloring are all basic design tokens used by all interface elements and can be divisive if changed.
Having said that, while I agree with the basic premise of the original discussions I would try to make changes in smaller increments to check adoption, bugs and complaints.
One changing aspect in particular that I think is very welcome is the increased contrast between regular text and headings. Before, it was subtle and made the visual scanning of the document more difficult, as @amilica previously noted with the screenshot below. The scaling of text I would not change initially as the bold text already solves the contrast problem.
This particular change I’ve had to do in the past in an intranet made with XWiki.
However, I would restrict the changes proposed just to only these headings for now and only on the content area, making them bolder by default like the example. Changing it in the whole UI can lead to unforeseen consequences so it would need to be thoroughly tested, and probably would lead to other changes in the interface as well (spacing for example).
Regarding Open Sans, we are in a weird spot. The font was declared but not provided, so only systems that had the font installed could see them, otherwise the browser would change the font to any alternative without serifs.
One way we could go forward is to embrace this behavior and use something like the System Font Stack. Booking and Zendesk are products that use it and have documented their journey in this area.
But I know it kinds of leave us in a similar spot. Some adaptation on part of our users would be necessary, some control on visual design would be lost, but also some gains would be made in the performance aspect and visual recognizability of the default OS font.
In a very particular opinion I would go for the system font stack, after a period of instability I think this would leave us in a better spot regarding visuals and performance.