Is there a way to change the font of an entire Google Slides presentation all at once? If not, this would be a very nice feature to have, and I suggest that google add it. [Note: I am not asking about how to change the default font, because that does not apply to existing text.]
Yes, there is, by using the Google Sheets API. Maybe the easier way to use it is by using Google Apps Script.
It's possible that there is a Google Slides add-on that does this. If it already exists, you could find it from the Google Slides toolbar by clicking on the Addons menu then on Get addons.