The general guidance is to migrate early if a question is offtopic. Generally, once answers have built up migration is discouraged - partly due to the issue you have mentioned, but also as it causes disruption with users on the other site losing rep etc. There are a few meta.so questions on this topic.
In general we do tend to see them come over without too many answers (ideally zero) but I think there will always be the odd one that gets answers and votes before being flagged and migrated here - I wonder how big a problem it is in real terms.
From an SEI perspective, you do want people to come here to find the 'right' answer and they tend to believe the votes, so there should be mileage in adjusting where the votes tally is not from specialists in the field.
An option is to delete the answer delete the answer - which could be fine if it is entirely wrong, but probably not ideal ifthe answer has good points in amongst the bad.
Another is to edit the answer to make it rightedit the answer to make it right - this should be encouraged. I don't think we do it enough here on sec.se.
