As far as I'm aware, cordon or espalier fruit trees are pruned in the early days to create the right shape, but it might be worth your ringing Keepers Nurseries - they're a fruit tree specialist, and if you tell them you either bought your current trees from them, or are thinking about whether you should replace those you've got because of the necessity for keeping them small, I bet they'd advise you whether its possible to retrain the trees you have, I've always found them very helpful. Otherwise, give it a go, see what happens - you can always replace them with something more suitable later on, either something pretrained or a dwarf version of fruit trees, such as patio or Ballerina apples.

If they are not near enough to the wall already though they probably wouldn't stand moving though if they have been in more than a couple of years.