*I add a second answer since I did not discuss policy in my first answer; mainly as I was a bit rushed and had not really looked into the current situation, but also since it was discussed somewhat recently in a different guise, but it is true this is a recent-ish feature so there might be some need for information and discussion.*

In brief:

Questions that are answered (well) already should in general not be migrated. 
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Here, are two complementary reasons for this. The latter is a direct consequence of old MO policy and  the former is just SE guidelines (which we should *respect at least in this case* as this action does not only affect MO, but another site in the network, and to say MO has its own rules thus does not seem a good position).

1. The SE faq have some information regarding migration; I would like to emphasize one point from there: http://meta.stackexchange.com/questions/10249/what-is-migration-and-how-does-it-work/10250#10250     


   >**Avoid migrating answered questions.** The point of migration is to send the question to an on-topic place when it can get answered. If the OP already has an answer, then we've already defeated the purpose of migration and the destination site won't have anything to do with the question. Avoid migrating these questions unless they are of extremely good quality and risk deletion on the current site.

   Also the other points given there are good to consider, please, see the end for my take. 


1. Migration has **immediate deletion of the answers** as effect, and eventually the full post is deleted. 
It is standing MO policy (and moderators intervene in cases of deviation brought to their attention) that questions with somewhat well-received and solid answer should not be deleted. 

  This was discussed at length and with great passion at various occasions. (Practical considerations in the way put forward in Stefan Kohl's answer played no role whatsoever since at that time the effort to delete a question with or without answers was identically, and it is not that different now either, as soon as one falls outside auto-deletion.) Personally, I sometimes wished that this policy was less strict and allowed for more deletions. But, the policy is at it is. To vote to migrate with *the aim* of answers getting deleted is *acting directly against a longstanding policy.* 

  Indeed, it seems even problematic to vote to migrate answered questions for any reasons as this deletion would be always a (possibly unwanted) consequence. But to give it as reason for migration is counter this site's policy on deleting answers (or rather on not deleting answers).

As alluded to in my comment and confirmed in this case it can happen that the status "answered well" arises only *after* votes were cast, and one can imagine some other exceptional circumstances, but as a general rule one should not migrated answered question.  

On the general matter of migration:

Three questions to decide if migration is appropriate 
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1. Is the questions off-topic for MO?

1. Is the question a good or solid on-topic question on the target site? 

1. Will OP benefit from  answers they would get (in addition to existing ones) on the new site? 


If the answer to the first two is not a clear "yes" do not migrate. And, if you do not assign a high probability of "yes" to the third one, rather do not migrate either.  

In general migrate only in clear cases. There is a custom off-topic reason for closing as off-topic and mentioning math.SE while not migrating. This exists for a reason. When only in slight doubt about migration, do not migrate and use this instead. It is really not much work to re-ask elsewhere via copy paste without migration, while migration has various issues to it (on which I do not want to elaborate as the answer is already a bit long).