The switch was from "first to invent" to "first to file". Prior art is still prior art, just the same in both types of "filings".

The "first to file" setup changed some of the critical date ranges around slightly that determine under what statute clause any given publication (i.e. prior art) may be used, but did not change the fact that a prior publication effects the patenting of a later application.

Now, getting that prior publication in the hands of the patent offices so they can utilize it to effect a later application, that is the hard part. Esp. if that prior publication was not somewhere big and well known (i.e., a personal blog would be a prior publication, but it is doubtful any of the patent offices would discover the personal blog on their own).

We switched from "first to invent" to "first to file," but published prior art still invalidates a patent.

The change just means that if two people try to patent something that they've invented privately, then the one with priority is the one who filed first.