Different agencies have different policies. Some just will not do it. I was surprised once when Trailfinders fixed it for me on an SQ flight to China, (with no obvious benefit to Trailfinders) but others just won't do it.
I was once told in the Corporate world that some travel agents pay for their tickets very late................but to get an upgrade, the ticket needs to have been paid for. I do not know if this is true.