I don't understand the relatively clueless response on the telephone, but all you need to do is send a letter request with a copy of your statement showing status.

If you can find a clued in telephone rep they may guide you on a fine point I have encountered in the past: a requirement for statements that show actual flight patterns in the past.

However, I suspect your situation is as they said above: letter with one statement, get gold now with platinum challenge, then platinum. You would lose Oneworld status privileges of the 50K tier for a few months I guess while you meet the challenge and get your new platinum card. For that reason and because of your CO status, I would REQUEST Platinum NOW with challenge to EP. Good Luck!