hi, it depends on the contract you sign, once apon a time all contracts were 12 months and you could upgrade once a year, but these days contracts usually last 18 months so the network operators get more money from you. when you go into the store ask the assistant how long the contract is for, however long the contract is for is when you can change phone. i have never upgraded a phone as the deals are very poor for existing customers, at the end of your contract you can dissconnect and port your number over to another network using a p.a.c. number ( porting authorisation number) that way you will get a better phone for free and still keep your number.