Currently when you use the various Generators or Makers, there is an opportunity to go back and edit the text you enter by using the "Change settings" button. I think if you use the Generate/Make button again during that workflow the old image is deleted and a new image is created.
If you don't use the "Change settings" button at that point, the opportunity with all your image settings is gone, and users need to start fresh.
Building something new to change text in an existing image is not impossible, but IMO it wouldn't really work well for images that were already submitted or images used in comments.
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