"what is the legal stance on one side of a party changing a legal agreement, (hosting and selling a product for an individual), without first consulting the other individual concerned?" That's actually par for the course around here, where have you been? Modifying a TOS is not the same as changing a contract/agreement. A contract is a binding set of terms and agreements. A TOS for a website in comparison can be considered a "whim".

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo