GREP styles cannot do this, for two reasons — you can filter on the en space and regular space at the start (why these?), but not “just on 1/8 inch indented paragraphs”. GREP styles don't “see” indents. The second reason is that GREP styles cannot apply a paragraph style (which would be a contradiction!).
Nested styles cannot “do” this either — also for two reasons. First off, a nested style has to be applied first, and if you do that, you might as well apply the correct style right away … Second reason is, as with GREP styles, a nested style can only apply a character style.
On “automatically”: the only 'automatic' formatting InDesign can apply is with a GREP or nested style, and the formatting it applies has to be a character style. InDesign does not apply paragraph styles automatically.
I think you should use a simple search-and-replace — search for paragraphs with a left indent of .125″, and apply the paragraph style in the Replace.
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