I posted an answer to question on SO which was then migrated to SU as #215180. At the time, I didn't have an account on SU, but I've just created one and linked it with my SO account. Will the system automatically link the answer to my account at some point, or should I ask for a moderator, or am I just out of luck?
You will get the rep for it eventually. If you are in a hurry you can request a recalc of your reputation by mailing [email protected].
But beware you might lose rep as well as gain rep if you have answered/asked question that has been deleted or migrated.