I'm not sure about the SEO impact but I can describe how it works on the technical side.
There is indeed no HTTP redirect. If you switch between 2 incompatible slug drivers, you will completely break your SEO because all the existing links will return 404.
The 2 slug-based drivers are compatible with each other though, because they only look at the ID at the start of the slug to match the correct discussion.
To a visitor it will visually appear like a redirect because our javascript updates the slug in the browser URL after the page loads and while it scrolls down. We can probably assume this is invisible to search engines since most don't run javascript, and even if they do they probably only look at the page content.
What search engines see is the <link rel="canonical"> head tag in the HTML, which will always contain the new slug for the discussion, even when accessed from the old slug. Search engines should treat this similarly to a 301 redirect since they see the page they accessed has a new canonical URL.
Also of note, whatever the SEO impact of this will be, is exactly the same as when you rename an existing discussion with either slug driver. Changing the title changes the slug, so the search engines go through the same steps as described here.
