That's still not clear. It doesn't say they will start charging for original quality shots taken before 2020. To me it just seems like if you want to keep storing original quality shots taken AFTER 2020, then you will have to pay for that going forward.

Assuming it's just like all of their other storage promotions have been over the last several years.

If you are over your free cap when the promotion ends, you don't lose access to anything, you just can't add anything new.

This. To add more stuff you change the setting. (When changing from "original" to the "high" quality setting, they convert your stuff to the lower quality.)

Source: did it before, probably still works the same way.

No. The photos that I uploaded in "Original Quality" are still original quality, and they remain that. I switched to 'High quality' because I don't have a Pixel, and it hasn't retrospectively changed the previous ones. That is also what Google probably means here.