For links to a migrated question, they will hold and will not break. When a question is migrated, the original question is soft deleted, just like any other deleted question on the site. As such, it still exists to allow for automatic redirection to the migrated post.
For example, https://stackoverflow.com/questions/3018462/ is a question asked on SO back in 2010 and migrated to Super User in the same year. The link to the original SO question still redirects to the migrated question.
Links in a migrated question (and content migrated with the question, like comments or answers) won't be affected except in 2 cases:
- Magic links in comments will break as described in Issue with magic site links in comments on migrated posts. These can't be fixed since comments can't be edited (normally) so the only option is deleting the comment or living with the broken magic link
- Relative links probably do not update, although I do not have any specific examples to demonstrate. The relative links will probably remain relative and will probably break since the system does not know where to redirect the user anymore. Given that relative links are extremely rare, there is actually little chance of it happening often enough to make it a concern.