Long range goal - replace a Foundation theme website with a Foundry one.
Are there risks by replacing each page of the Foundation site with a Foundry page? Each Foundry page would have the Master Style unchecked and Foundry theme selected in the Page Specific theme area of the Inspector.
Or is there a better way to replace an old site with a new one using Foundry?
Personally, I would never mix two framework systems. I would simply start a new, clean project.
But why don’t you make a copy of your project and start experimenting?
There should be no technical issues with having both frameworks in the same project as long as each page had it’s theme set appropriately. The main issue will be the inconsistent look during the transition.
If you’re switching from Foundation to Foundry, there’s really not much of a reason, IMHO, to build your new site in the same project file. As @Panans pointed out it would be less confusing and eliminate the possibility of slipping up and combining the two frameworks’ stacks accidentally.
He can avoid that by creating a new site file in Foundry and just drag those pages (as he’s finished) into the old Foundation project in the same spot as the old Foundation pages. That way he’s updating the site and, when he’s done, he just fully republishes the new site.
Here’s a quick video. NOW the important thing is that your pages that you move (from Foundry into the Foundation project) HAVE TO BE IN THE SAME FOLDER (in other words, set the Page settings up with the exact name and folder as you had the Foundation pages you’re replacing. Good luck.