Unfortunately the only fare rules that seem to apply are H, which are luckily much less restrictive than the R ticket is that I'd have ended up purchasing via LGW or MAN. I don't know offhand the D rules that would apply, but they are generally fairly open to changes.

I just tried a dummy booking since only J is available on the DUB-EWR flight on 3/14. It is pulling from J inventory although it's pricing at H.