If I already have section 8 can i move to another city?

Absolutely! This is known as porting your voucher for one jurisdiction to the other, and can be done nationwide. However, keep in mind that the initial Housing Authority - the one that issued the voucher - may not allow you to port if it cannot afford the costs of maintaining and administering your voucher, because unless the receiving Housing Authority is willing to absorb the costs, the initial Housing Authority will be billed.