I want to change the wording of the automated text message that my supporters get when they text in to order tickets, complete a survey, etc."
This message was set up and locked into place in this format due to regulations.
Right now, the only custom, automated message you can send is the one you can set up as a One Time Response. To locate the One-Time Response field, click Activity Keywords from the left navigation, then select a subscription activity.
For more on building an SMS Subscription activity, click here.
You can use the shareable link from any Activity with the SMS Subscription One Time Response message, but you will lose the functionality of the Fulfillment reminder text messages, which may result in less fulfillment rates.