Had a sked change, on UA metal which I accepted. It's a downgauge from 319 to CRJ, and I lost a GPU segment, but it can't be helped. My ticketed flight went from 'W' fare bucket to 'R' on the UG, and is now in the 'X' bucket.

Any insights into what this might mean for mileage or other issues down the line?

I'm not expecting this a change I'll like, but will suspend judgement until I hear from those who've traveled X fare categories.