I tested clicking on those permalinks. I was expecting either of 2 results:
- permalinks become dead links
- permalinks lead to new custom domain permalinks
What I got was a surprise, neither of the above. Clicking on the permalinks led to the old permalinks (individual post page URL) prior to conversion. The only clue that there is something strange is a little message at the task bar "Done but with errors on page":
I will check them later to see if the situation changes. Update: All pre-custom domain permalinks lead to the new custom domain permalinks seemlessly