Fixed mileage redemptions are ticketed on one single fare, so even your Y segments will have the "I fare" associated with the segment coupon, which entitles you to be seated in the cabin you're ticketed in.

The only possibilities of a mixed cabin market fare award are:
1) There is no J cabin for the segment you are ticketed in Y (this one is obvious); or
2) There is a fare break for your routing.

The second one is less obvious, let me explain: imagine you trying to fly YYT-YYZ-YOW, but a through J fare only has space in C/D/J and no P/Z spaces anymore (due to married segment logic). The market fare for this all J itinerary would require insane amount of miles. You happen to find an itinerary that puts you in Y for the YYT-YYZ segment and then J in the YYZ-YOW segment that costs only half the miles compared to the all J itinerary. This is because your YYT-YYZ portion is ticketed in the Standard fare class and your YYZ-YOW segment ticketed in discounted J class. The fare is broken and your YYT-YYZ has an economy fare basis associated with its coupon while your YYZ-YOW segment has a business fare basis associated with its coupon. As a result, even if there are spaces available in J for your YYT-YYZ segment on the day of the flight, you cannot upgrade to J cabin without using one of the upgrade instruments (bid upgrade, LMU, eUp with co-pay, etc).

Hope this helps.