You are right. The migration process has changed the URL of all posts. However, it has taken all the user provided links inside a post as links to external sites, and did not update these. As a result the links to other posts are broken now
Example:
Below is the original URL of TWEAK-1 post.
I have added this URL in other posts before migration: discussions.flightaware.com/post209050.html#p209050 (Click it to see what happens)
My added URL did not change during migration. It is still the above one, while migration process has changed the URL of the TWEAK-1 post as below:
discussions.flightaware.com/t/tweaks/20339/2 (Click it to see what happens)