I agree that that the doctrinal implications of the calendar change are subtle. It's not like they just added the Filioque to the creed; that would have been obvious. If you want to know what the GOC thinks about the doctrinal implications, you can consult our original 1935 confession of faith:http://www.ecclesiagoc.gr/e_index.htm
(look under "History")
The basic line is that the calendar change is a doctrinal matter in that it violates the teaching we confess in the Creed "One Holy Catholic and Apostolic Church", since that unity is violated when we do not celebrate the feasts and fasts on the same days.
Whether or not the calendar change was a doctrinal issue in the strictest sense, the Western calendar had been anathematized several times by the Eastern Orthodox Church, not only the Paschalion but also the Menologion. And, as you say, the calendar change was not implemented by an Ecumenical or even Pan-Orthodox Council, which might have had the authority to do so, but by an 'Inter-Orthodox Congress', whose authority was not at first recognized outside Constantinople, Greece and Romania. Fr Basil Sakkas, a Greek Old Calendarist under ROCOR, explained all these issues in his book "The Calendar Question", available here:http://roacusa.org/Catechism/THE%20CALENDAR%20QUESTION.pdf
This short book has a very significant foreword written by Met Philaret of New York, which is incidentally yet more evidence for the great shift in ROCOR's attitude from pro-Old-Calendarist to pro-New-Calendarist as we find now.
I will quote a short section from the chapter on the doctrinal significance of the new calendar:
Our adversaries pretend that the calendar “is not a dogma,” thus leaving it to
be understood that one can do with it what one pleases. Is the question of the
calendar truly one of dogma? This naturally depends upon the perspective from
which one examines the matter. My beard and my rassa certainly do not constitute
a dogma, for their existence does not increase or decrease the number of the
Persons of the Holy Trinity. However, if I disdain the insignia of my ministry with
which the Church of Jesus Christ has honoured me -- which She regards as more
precious than royal purple -- will I not thus offend the Church Herself? Though my
rassa and my beard do not in themselves constitute a dogma, yet, if I take them off
without any reason, do I not dishonour the Church which has honoured me and
which is the foundation of all the dogmas? How, therefore, is it possible to isolate
the dogmas from the rest of the life and the experience of the Holy, Catholic and
Orthodox Church of Christ?
For this reason, Synesius, the Metropolitan of Cassandria, speaking of the
State (i.e. new calendar) Greek Church, says with justice: “The Greek
Autocephalous Church is independent. For us, the very thought of the abolition of
the celibacy of the higher clergy and the alteration of its clerical dress is very
premature. Today, these two questions have nearly become dogmas and cannot be
removed. Consequently, there can be no place for any official or unofficial
discussion of this matter” (Ecclesiasticos Agon).
Thus, the dogmas are not clearly independent of the details of the daily life
and acts of the Holy Church. It is nearly impossible to make a distinction between
the primary and the secondary in matters of the Faith. All these things bear the
sanctifying seal of the Holy Spirit, to such a degree that we cannot touch the least
matter of the Tradition without directly or indirectly disparaging the Church’s