On this hypothesis, both are fully flexible fares: you can change without a change fee. But neither fare is completely free of restrictions on changing. In other words, thinking of "fully flexible" as meaning "I can change without restrictions" is fallacious - and equally fallacious in the case of both fares. The R class fully-flexible fare is likely to be more restrictive than the J class fully-flexible fare, because R is more likely to become unavailable than J class - but that's a question of degree, not a difference in principle.