AT&T is doubling its price to upgrade from $18 to $36 tomorrow. This means if you have an upgrade you were planning on using in the near future you might want to get it done tonight. As of tomorrow it will cost you $18 more to activate your phone. AT&T claims the increase was necessary due to the increased sophistication of smartphones today. This doesn't exactly seem to be a valid reason as most activation work is automated and it really doesn't cost much money to swap out a SIM card.
Either way they are going to be charging more and if you have an upgrade ready you might want to consider going ahead and using it to save you a little money. If you need an idea of which phone to buy check out my Top 5 AT&T Smartphones Not Made By Apple.