If "processed" means actually resampling the file, then definitely yes, most of the detail is gone after resamping smaller. Not recoverable except from the intact pristine original copy. It is a good argument to always save a resample as another file copy, and also keep the original intact original image file.
300 dpi at 8x12 inches is 2400x3600 pixels, 8.6 megapixels.
160 dpi at 6x9 inches is 960x1440 pixels, 1.4 megapixels.
About 15%. The rest of the pixels and detail is simply gone now. You might enlarge it, but you will never get the lost detail back. It's gone.
But if "processed" only means Walmart printed it, then the original file should still be intact, if you still have it. In that case, to know what it is, you would want to check its image dimensions, in pixels, now. It's image dimensions, in pixels, is all that matters.