The general rules on what are 'good' migrations are that the question should be
- on-topic on the target site
- not on-topic for the 'source' site
The last point is important in the sense that the original poster has made some form of choice of where to ask: it might be accidental, but it might equally be deliberate if the question is arguably on-topic for more than one place.
The case of Emacs and specifically AUCTeX questions falls at least partly within the 'TeX-related editors' area that we have been happy to have on TeX-sx. However, that's something of a compromise: we know that sometimes the 'TeX element' is more-or-less zero, but with no better place to point to we've preferred to give people the benefit of the doubt. The new site means we at least need to think about the balance.
I'd say that we can't make a blanket 'questions will be migrated' decision, at least at this stage. What we can do is look out for things that make the question more likely to be answered on the new site. Most obvious there would be ones that look likely to have general Emacs answers (i.e. the fact that TeX is involved is parenthetic) or where there is an element of Emacs programming involved. Less clear are those tied to AUCTeX where there is a strong 'TeX element': they are likely to be similar to questions asked about other editors where we would allow them. I'd be tempted in such cases to point the OP to the new site and suggest we can migrate if they want. That leaves the OP with some 'ownership' while still trying to achieve the point: get them good answers quickly.