[Atrix 4G Q&A, Help & Troubleshooting] edgeicator:
Wrong. Do not believe this post. Your ToS explicitly states that tethering without a tethering plan is against the ToS. It is you who violated the ToS.

AT&T provides wireless data and messaging Services, including but not limited to, features that may be used with Data Services and wireless content and applications ("Data Services"). The absolute capacity of the wireless data network is limited; consequently, Data Services may only be used for prescribed purposes. Pricing and data allowances for Data Services are device dependent and based on the transmit and receive capacity of each device.

On Data Services with a monthly megabyte (MB) or gigabyte (GB) data allowance, once you exceed your monthly data allowance you will be automatically charged for overage as specified in the applicable rate plan. All data allowances, including overages, must be used in the billing period in which the allowance is provided. Unused data allowances will not roll over to subsequent billing periods.

AT&T data plans are designed for use with only one of the following distinct device types: (1) Smartphones, (2) basic and Quick Messaging phones, (3) tablets, and (4) LaptopConnect cards, and (5) stand-alone Mobile Hotspot devices. A data plan designated for one type of device may not be used with another type of device. For example, a data plan designated for use with a basic phone or a Smartphone may not be used with a LaptopConnect card, tablet, or stand-alone Mobile Hotspot device, by tethering devices together, by SIM card transfer, or any other means. A data tethering plan, however, may be purchased for an additional fee to enable tethering on a compatible device. An Activation Fee may apply for each data line.

Consumer data plans do not allow access to corporate email, company intranet sites, and other business applications. Access to corporate email, company intranet sites, and/or other business applications requires an applicable Enterprise Data plan. Enterprise Email requires an eligible data plan and Device. Terms may vary depending on selected Enterprise Email solution.

AT&T RESERVES THE RIGHT TO TERMINATE YOUR DATA SERVICES WITH OR WITHOUT CAUSE, INCLUDING WITHOUT LIMITATION, UPON EXPIRATION OR TERMINATION OF YOUR WIRELESS CUSTOMER AGREEMENT.

6.11.1 AT&T Data Plans With Tethering Print this section | Print this page

Tethering is a wireless or wired method in which your AT&T mobile device is used as a modem or router to provide a Internet Access connection to other devices, such as laptops, netbooks, tablets, smartphones, other phones, USB modems, network routers, mobile hotspots, media players, gaming consoles, and other data-capable devices. AT&T data plans with tethering enabled may be used for tethering your AT&T Mobile device to other devices. If you are on a data plan that does not include a monthly megabyte allowance and additional data usage rates, you agree that AT&T has the right to impose additional charges if you use more than 5 GB in a month; prior to the imposition of any additional charges, AT&T shall provide you with notice and you shall have the right to terminate your Service (early termination charges may apply).

So, no. Them changing your data plan will NOT, I repeat NOT, allow you to get out of your contract early.