Contract terms and Data Cap

My contract dating back nearly 15 years does not stipulate a data cap. I have not signed another contract since that would be subject to a data cap. It seems unethical and the legality dubious to arbtrarily change terms of contracts without having both parties agree to the terms. 

If Xfinity wants to change the terms of my contract by modifying the service itself then I should receive a copy of a new contract with the terms and if this is to be a new contract then new equipment should be in order after nearly 15 years.

Thank you.