Is booking class J flexibile? If I try to make test booking VIE-LHR-JFK-LHR-VIE with flexibile conditions, it shows that VIE-LHR and LHR-VIE are in booking class J.

However, I have booked same trip for August but in different booking class for sectors LHR-JFK-LHR (I). European sectors are in J class so I believe that it should be flexibile.

But when I try to change my reservation in MMB, in fare rules it shows: "Changes: Time/date changes permitted at any time before each flight departure for a change fee of €350 or an upgrade fee of €350 plus any difference in fare. Changes subject to availability. Fees apply per ticket"

So my question is - is it bug or I can't change my LHR-VIE sector because JFK-LHR sector is in I class?