I've now bought the $200/month plan from OpenAI and simply ran `/status` in Claude to get my session ID, then asked Codex in the same directory to "Please take over the work started by Claude Code with session ID `[…]`." This seems to work like a charm. It also seems that the Codex weekly and monthly budgets are more generous?
Unless Anthropic adjusts their customer-abusing behavior I think I will end my subscription with them soon.