For full context: the Markdown source of the text in your screenshot:
> Access denied with the following error:
> Too many users have viewed or downloaded this file recently. Please
try accessing the file again later. If the file you are trying to
access is particularly large or is shared with many people, it may
take up to 24 hours to be able to view or download the file. If you
still can't access a file after 24 hours, contact your domain
administrator.
> You may still be able to access the file from the browser:
> https://drive.google.com/uc?id=1LC5iVcvgksQhNVJ-CbMigqXnPAaquiA2
There was no automated migration done for that answer. It was your edit that resulted in the answer being re-rendered.
First, remember that when a post is made or edited, it is only rendered once, and the HTML from that render continues to be served even though changes are made to the renderer. Re-editing a post will cause it to be rendered again, using the current version of the render. (This gets confusing when looking at revisions, as those behave differently and always render with the current version, but that's besides the point here.)
When the system went through and performed automatic edits to posts as part of the CommonMark migration, it checked to see if its automated edit would result in the rendered HTML being the same as the previous cached HTML from the last edit, made under the prior renderer; if it couldn't automatically edit the post so its rendered HTML would be the exact same, it left the post alone and didn't perform any automatic edit, and so the previous rendered HTML would continue to be served.
The rules list for that automated script was extremely basic, however, covering only the most common issues (e.g. ##Headings, block quotes without continuity, etc.), and excluding subtle nuances. This did, however, mean that if a post did contain such a subtle nuance, no matter how minor, it would prevent the post from being edited by the script, and so other issues that were covered by the script wouldn't be fixed.
In your case, the markup above uses two things that were compliant with the older renderer, but not with the new one:
Non-contiguous block quotes. Under the old renderer, two lines with block quotes would always render as a single contiguous block quote, regardless of blank lines between them. However, under the new renderer, those will be treated as two separate block quotes rather than being combined into one:
> This and the below line would previously be rendered as a single contiguous block...
> ...but today render as two separate blocks
To get them to render as a single block, you now need a right angle bracket between the two lines:
> This renders as a single contiguous block...
>
> ...fully compliant with the new CommonMark renderer.
This rule was common enough to be included in the migration script, though. But, as your post wasn't automatically edited, what happened here is that there must have been something else that wasn't compliant that caused the automated script to back out. And, after looking at your post, I can see that it happens to be...
Multi-line code block within a block quote. Under the prior renderer, if you wanted a code block within a block quote, it was only necessary to preface the first line with a right angle bracket. However, under the new renderer, every line needs to be prefaced with right angle brackets:
> This would render as a single code block under the prior renderer, ...
...but wouldn't under the new renderer...
...because not every line is prefaced by a right angle bracket.
The following works under the new renderer, however:
> This will render as a single code block in the new renderer, ...
> ...because every line is prefaced by a right angle bracket, ...
> ...which is now required.
In summary, your post wasn't compliant with the new renderer, and contained a problem that wasn't common enough to be included in the automatic migration script. This resulted in the old cached HTML from the previous renderer continuing to be served, which masked out the problems for a while, but as you just edited your post, the problems are now fully visible.
