I think it all depends on which leader they get. As far as I know, of the potential candidates only Jon Cruddas would support electoral reform. And he is unlikely to win.
Ed Balls and Ed Miliband are definitely against change, and I imagine David Miliband is too. I suspect Alistair Darling is against as well. I imagine Labour will largely campaign against despite their AV pledge in the manifesto, with a number of notables campaigning for - such as Alan Johnson and Peter Hain.
I think Labour's position on this will also depend on what is planned for constituency boundaries. On the present boundaries which are likely to be changed before the next election, Labour would do well out of AV, with the Tories doing worse. With boundary enlargement I suspect the Tories might pick up 2nd preferences from more rural Lib Dems and do much better.