Hello.
I am entirely new to this site, so if someone has made this point apparent before, I am sorry, but I did not see it while looking for it.

For my comic, I made the rating 'E' without thinking. Is there a way to change it to something higher, or must I scrap it entirely and start anew?

I had to change it to. You have to go in "My Account" and then "Comic Overview" there is a blue link named "Show Advanced Comic Descriptions" click on it and you will get the rating option.

If you want the new rating to be shown on your thumbnail you may have to reload it.