Not new. AA's insistence to keep isolated fare buckets certainly seems unnecessary sometimes, especially in your case where they cancelled your flight.
It drives me nuts on SDFC, where the original fare needs to be available. So there needs to be an A fare seat available to switch if you have used miles copay. Give me a break.
How does it help them with revenue to force you to stay in one plane, rather than allow you to switch flights? No idea. Also seems agents don't have the discretion they should be given to make these sort of changes.
With Twitter, you might ask them to forward your flight change request to the Aadvantage desk? (Others might know, who has the final say on whether to open an award seat?)