Logic says you get carried in J and the ACIR26 fare specifically excludes any compensation for accommodation in business class on a flight that does not offer first class.
When I had a F award booking with CX and they removed F from the route I was rebooked into business class. As there were still other F segments in the itinerary, no adjustment was due for the cost of the award following the involuntary downgrade. I expect the same would be true for an ACIR26 fare.
CX tends to make a distinction albeit unofficial of when you are booked and ticketed in J vs booked and ticketed in F but invol downgraded at the last minute. Anecdotally compensation is given.