No, not unreasonable. Even if "legally" they can do it, doing so without letting the customers know just isn't a great way to do business. If they say on their advertising and packaging that "substitutions can/will/may be made", then it still would be a nice thing to do- tell the customer before they buy it! I think I would discuss what other options might be available to you, they may offer a discount, another sub, or something else. I would definetly discuss it with them.