Best time to do it is when dormant and leafless.

I would think your best bet would be to contact a local nursery, as long as they can access the tree with their equipment. However, even dormant, a 6-8 inch diameter tree would be challenging. I agree it would need to be done in the winter/early spring and I suspect you would need to prune it back quite a bit.

rkru:

For what you will spend to hire moving a mature apple tree, you could buy enough new bareroot trees to plant an acre. And there is always the risk the move will not be successful. I don't think anyone would guarantee survival after moving a mature tree. Of course, if price is no object.....

If it is the characteristics of this particular tree you are trying to preserve, or it is a variety that is rare or not available, you could take cuttings and try to graft to a size-controlling rootstock. Of course, cuttings would have to be taken in the dormant season.

Don Yellman, Great Falls, VA