There are about 2011 old (La)TeX related questions in SO. Is it necessary to migrate them to TeX.SE? Will it be a heavy task?
I can support this but only if...
The question is not just good, but GREAT
It is STRONGLY on topic at the target site
You feel VERY STRONGLY it belongs here
If all of those criteria aren't met, it is probably best to let the question stay where it is.
No. It's always been clear that the existence of new SX sites does not mean that there is no overlap with the main site. For example, questions on the detail of LaTeX programming may well fit within the remit of StackOverflow. At the same time, it seems pretty clear that the idea was never to 'flood' new sites by migrating questions which are no longer active. There is no real gain for anyone.
In my opinion, migrating old LaTeX questions from SO to tex.sx is a bad thing in many cases. The reason is that -- because of SO's higher number of users -- these question are on a quite different scale with regard to upvotes and number of views. Case in point: This migrated question has the second highest number of upvotes at tex.sx (66), has generated the answer with the highest number of upvotes (87) and has about 39k views. (The only other question with >10k views at tex.sx is this one which nearly caused a flame war.) Mass-migrating of old SO questions will completely destroy tex.sx's benchmark of what "Good questions" and "Good Answers" are. Don't do it.