Clarification from IRS. You can recharacterize a 2017 conversion. I think this is "news".
"How does the effective date apply to a Roth IRA conversion made in 2017?
A Roth IRA conversion made in 2017 may be recharacterized as a contribution to a traditional IRA if the recharacterization is made by October 15, 2018. A Roth IRA conversion made on or after January 1, 2018, cannot be recharacterized. For details, see “Recharacterizations” in Publication 590-A, Contributions to Individual Retirement Arrangements (IRAs).