Yes, you can get a new phone at any time, but if its before 18 or 20 months (depends on market.), you will have to pay full retail price for the phone you want. You can also purchase a used phone which is usually 50% off of the retail price.

At the 18/20 month point, you can renew your contract and get the phone at the subsidized pricing.

You can now renew your contract at any point in time to take advantage of new promotions, but you cannot get a discounted phone until you are 18/20 months through.