False advertising is false advertising. If I buy a basket of apples, and under the top row it's all watermelons, it doesn't matter if I think watermelons are a fine fruit too, the point is I bought a basket of apples.

Honesty and integrity in advertising is important. Don't sell a product, and then stealthily change what product it is you're selling. If you need to make substitutes to your product, announce it, offer refunds, and understand that not all customers may want your new product.