The purpose of the customizable field titles is to help guide the user to enter the right content into the right spot, not to confuse the user when the field suddenly has an unrecognizable name. I recommend keeping consistent field titles and adding the customized field title next to the standard field title.
Currently it works like this:
I recommend instead using the HTML5 placeholder attribute to show the text that should be entered. Note that the fields are now Title and Subtitle.