Some I fares have no change restrictions as long as I fare bucket is available. Restrictions on different I fares vary considerably depending on the exact fare bucket used and the origin. In each country there are numerous I fares and coniditions can be quite different. However typical corporate I fares do not have much change restrictions except that I fare bucket must be available. Otherwise you have to upgrade to the corporate D fare. Companies need flexibility for their business pax.

Despite the wording "company agent", it would seem surprising that a corporate I fare was used as there is plenty of I availablility on the desired flight but the TA would not accept to change it.
I asume that the "company agent" wording by the OP does not mean that this was a corporate fare, but rather a leisure fare offered by the TA. I frequently use travel packages exHKG in business class (companion tickets including hotel) and those I fares are totally nonchangeable/nonrefundable. That could be the case here.