Can at&t data plan change without my consent?

I’m sure it can be done. According to the AT&T consumer policy this is possible.

It is clearly mentioned in their plan catalog that If it is firm that you are using a voice-capable Device with no voice plan, or that you are using an iPhone or designated Smartphone lacking an eligible voice and tiered data plan, AT&T reserves the right to exchange your plan to the required plan or plans and charge you the suitable monthly fees.

In the case of the tiered data plan, you will be located on the data plan which offers you with the maximum monthly data usage payment. If you determine that you do not need that much data usage in a month, you may ask for a lower data tier at a lower monthly recurring fee.

I hope this information will resolve your problem.