Warning: saaS provider changed terms unilaterally

Has anyone dealt with something like this? I'm not sure what my options are.

SaaS provider changed terms unilaterally. I've been dealing with this for about 3 months now and the situation isn't improving.

The contract was signed 16 months ago. I am not sure I have the original signed copy. The total amount in dispute is approximately $138,000.

Do I have a strong case? What should my next steps be?

Just want to point out — the statute of limitations might be a factor here. In some states it's as short as 1-2 years. Don't sit on this too long.