Of course things change.
We know this, you are misinterpreting what we say about Holy Orthodoxy. Much of this is organic, and usually it was in the order of discipline, not theology.
As for the advancements in theological understanding, they were not change for changes sake. The early councils were engaged in paring back, pruning away the developments of heretics who were the innovative thinkers using logic and philosophy in their teachings.
For us sometimes that means positively affirming beliefs that were always assumed by the core orthodox Christian community. These ideas are usually found in the prayer life of the community, directly in the liturgical practices.
Remember what
“define” actually means: it is the establishing of limits. A very apophatic concept.
We have not needed to make any major declarations since we all (your church was involved in this too) affirmed the Holy Spirit was God and Christ had two natures. These were inherited wisdom, not exercises in logic.
Christianity is a Faith of revelation. We only wish to know what Jesus passed on to the Apostles, there is evidence of that in the community. Beyond that can only be educated guesswork by fallible human creatures.
We do not glorify development, and do not pursue it. We do not use the tools of philosophy or logic to extend or stretch our understanding and when that is attempted by others we do not think of it as wonderful. This is a major difference between us.
What organic changes in practice we have experienced have been very slow in coming. If you read the writings of St. John Damascene (a Doctor of your church 7th-8th century) you will find that he very clearly describes Orthodoxy as we know it today. He is not just an example of an earlier phase of Christianity, as he might seem to you.
We
still follow the ancient
canons of the seven councils. They are in full force and applied in the same manner as they were when composed. These were superceded long ago under the Popes by an edited and redacted
Code of Canon Law.
Respectfully, I think there is an entirely different attitude and approach to change between our two churches, yours embraces change much more readily, even eagerly, feeling confident in doing so. Ours embraces the formal decisions of the early Fathers much more strongly and stubbornly, we are not convinced of the value of further changes.
Michael