espalier fruit trees

Good morning, Following the advice from people contributors on here, I'd like to have some espalier/cordon fruit trees in a walled garden. In the garden there is already 3 fruit trees ( one cheery one apple one pear ). They are about 7ft and have never been pruned properly so they're not very productive. Is it possible to make them into espalier or cordon or is it best that I start again? If so, how? Many thanks, Sam