USB devices are designed to use a wide variety of voltages ranging from 500mA to 1500mA and have been standardized for quite some time. You can generally plug any USB device into any USB cable and into any USB port, and nothing will blow up. Using a more powerful charger should speed up battery charging, while using a less powerful one will just charge slower. A good resource for this topic can be found here.
As noted in that article, the only real concern is when charging an older phone with a newer charger, as the battery might not be able to handle the higher current. In your cause your using an older charger with a new phone, so your all good.