Is changing a promised gift considered a breach of promise, and what should I do if I fear the gift will be used for something unlawful?

There is no harm in changing a gift and exchanging it for something else. A gift is not binding until it is taken possession of. If you have not taken possession of the lens, it is not binding on you. Furthermore, fulfilling a promise is recommended but not obligatory. If you fear using the lens to photograph impermissible things, then give her something else as a gift, and you may inform her of the reason for the exchange.