Today AT&T updated its store trade-in program to incentivize more customers to trade in used smartphones for an upgrade. The carrier is now giving $100+ credits towards the purchase of a new smartphone with a two-year plan. The catch is that you have to trade in a smartphone that is “no more than three years old and in good, working condition.”

If a new phone purchase would normally be $199, then this credit would cut the upfront cost in half. A $99 phone could be purchased for free. If the trade-in value of a newer smartphone is higher than $100, AT&T will give a larger credit.

This looks like a limited-time deal, but AT&T hasn’t said when it will end. You can take advantage of this offer starting tomorrow in all AT&T stores.

Beginning May 1, consumers who trade in their current smartphone to AT&T will be eligible to receive at least $100 off the purchase of a new smartphone, including the new Samsung Galaxy S 4, BlackBerry Z10 or 32 GB HTC One. These devices are normally $199.99 with a qualifying two-year agreement** but, with this limited time promotion, they will be available for $99.99.

This promotion applies to any smartphone AT&T sells, and gives customers access to the latest devices at a fraction of the cost or, in the case of smartphones priced under $99.99, for free. To take advantage of this offer, customers are encouraged to visit any AT&T company-owned retail store or participating authorized dealer with their previous smartphone that is no more than three years old and in good, working condition. In stores, upon trading in their old phone, customers will receive the $100 credit on-the-spot and can use it immediately.

Some trade-in phones may have a value higher than $100. In this case, customers will receive the credit at the higher value.

• SourceAT&T