I read something that suggested that a phone could be unlocked at the factory or by a owner. Maybe I got it wrong. But if the phone is unlocked by the owner...let's say he has an AT&T phone and takes the Sim out (is that the way it would be done?) and then sells it on to someone who wants to use another GMS network such as T-Mobile or Boost Mobile or Cricket, is there going to be a conflict with app updates? Stuff that AT&T installed as part of a proprietary version of Android, for instance? How often are "unlocked" phones actually "factory unlocked?" Apologies if this is a stupid or odd question...but it bears on buying a used phone.