Although there is an attribute style called lowercase, in a sense, there is no such thing as the opposite as a style or attribute because InDesign cannot read and know when a word should be capitalized. But you can Find/Change Heading text to be typed (a new addition called Change Case) in Sentence Case, or Title Case, or lowercase. Next, the Heading style could be edited to apply the appearance of all caps. This basically gets you back to where you were before, except that it is now built correctly and typed correctly. Lastly, the ToC could be refreshed and it would pull in properly typed upper/n/lowercase characters.