You could adjust four of the forty-odd emails. Now there are nine of them, and you adjust more than just the middle paragraph.
What is new
Under Management > Customer messages you will find every message your customer gets from you, with the fields that message has per message:
Subject
Preheader
Intro and closing
Button text
Not every message has all five. That is not an omission: the proposal for a new appointment has no branding block, for instance, so there only the subject and the preheader are yours.
You see it before your customer does
Next to every message sits View example, and that example runs through exactly the same build-up as the real email. What you see in the example is what gets sent.
If you want to see it in your own inbox, Send test email sends it to your own address. Not to an address you type in: that is deliberate, because a button that sends arbitrary text to an arbitrary inbox from our sending address is something other than a test button.
A test email does not end up in a customer's message history. That history is what your customer really received, and a proof copy to yourself does not belong in it.
What you do not lose
If you had already written an intro or closing of your own on one of the four emails that allowed it, it is still there. Those texts have moved along to the new screen.
The default text is in there as grey placeholder text, not as a filled-in value, so you can always tell which words are yours and which are ours.