Simple tip modifying the object style associated to the standard text block... but need to see screenshots of your pages.
Hm, I haven't worked with GREP at all, so I'm not sure how this is going to work?
Will I format my text with "Heading" and "Body w/ indent" as next style and then use your GREP to change the first paragraph after the heading to "Body"?
Select the headline and before the next headline and apply Para Style and next style (assumed that the second style has next style is the same style.)
No! Exactly as explained in my post.
If all your text is actually in the same para style, apply Body first, then apply Heading.
Then, you can play the regex to treat all the paras you want to be treated with "Body w/ indent" para style:
Search format: Body
Replace the format: Body w/ indent