You do not change the actuall amount of extract you use but you do change the amount of preboiled water. You have to account for more boil off when you start doing full boils. For example, a five gallon batch I would start with 6-7 gallons of water....
if you have a way of cooling 5gal of wort, by all means, do a full boil... I use an ice bath, which isn't a very good option for chilling 5galons, so i still do partial.