I believe you may be wrong about this. The unlimited data plan on AT&T was made for smartphones. They took it away before they allowed tethering on the iPhone. When they came out with a tethering plan they made it clear that customers who wanted to tether needed to be on their tethering plan. They have already told customers they need this plan. Some, who jailbreak, do not listen to this, and as such, I don't even think AT&T should be sending warnings to these people, they should just switch them.
BUT to be fair, AT&T did send the text about two months ago saying to call them to discuss the situation. If someone is getting the second warning that they will be changed, I don't think AT&T needs to offer them a date it will be changed, it should just be done.