All votes should be reset when a question is migrated. That's votes on the question, in both directions, as well as votes on any answer.
At the moment, downvotes are cleared from the question. Yes, it makes sense on its own, but I don't think this is the right solution.
In my experience, communities vote very differently. A mediocre answer can get a lot of upvotes just because no one on the source site knows better. An easy-looking system installation question on Stack Overflow is sometimes a near-NaRQ on a more appropriate site like SU/SF/U&L, and yet it arrives with plenty of upvotes.
Two migrations in particular triggered this post: 1 2 (discussed here). Now these are old questions which presumably shouldn't have been migrated — but they were, by a moderator, and they are indeed on-topic on the target site — but the votes don't fit.
Browsing the list of questions migrated to U&L, I see that a majority aren't a problem — because they came with very few votes from the original site. So retaining the votes during migration doesn't contribute much. Then why bother?
Please give migrated questions a new life. They might come with answers attached, but let the target community decide what they like. Reset all vote counts on migrations.
Related: and make migrated questions as visible as new questions. For example, Make migrated posts be part of chat RSS feed.
Adding support from IT Security.
Leaving this issue unresolved can especially have adverse effects on younger, smaller sites when the migrated question comes from one of the big ones like SOFU or Programmers.
On the IT Security site, nearly half of the top ten questions by votes are migrated questions:
- #3: What alternatives are there when SSH is being actively filtered?
- #4: What should I do when my boss asks me to fabricate audit log data?
- #6: Do any security experts recommend bcrypt for password storage?
- #9: How can I avoid my password being harvested by key loggers from internet cafes?
There's also five more within the top 50.
Most of the above questions have answers with up-votes carried over from the origin site, which put them (in some cases) inappropriately much higher in the thread than more accurate answers provided by the receiving site. Meanwhile, the receiving site does not have enough active users to counter the relatively massive number of existing votes where it is needed.
Same experience on Computer Science
Many computer science questions are posted on Stack Overflow, and get answers which don't really explain why things are the way they are, or misunderstand the issue. These are poor answers from a computer science point of view, yet the question is a good one and would benefit from a computer science point of view. The ideal fate of such question would be to migrate them from SO to CS, where they will get better answers — but that is not viable if the upvotes on mediocre answers are retained.