1) you will have to recharacterize the roth contributions that exceed whatever your income cap is. I had to do this with fidelity and it was relatively straight forward. Although it kind of jacked up my taxes a bit. If you've invested that money, you'd have to pay the taxes on the appreciation. After the money has been "moved" to non-roth aftertax IRA, then you recharacterizeconvert that money back into the roth.
Fixing the language slightly.
OP -
Normally, you can recharacterize before your tax filing deadline.
However, you want to also do a conversion for tax year 2018. Conversions have to be done in the same calendar year. So your perfect execution looks like this:
1) transfer existing IRA money into a 401K so as to not screw up your backdoor Roth contribution
2) recharacterize your Roth IRA contributions to non-deductible traditional
3) convert the non-deductible IRA contributions to Roth
1 and 2 can be done in any order, but both must be done before 3
and that has to be done by 12/31.If rolling existing tIRA money into your 401K isn't an option, I'd convert all tIRA money and pay tax on the 10K to get yourself better set up going forward. Unless this is an unusually high income year, setting yourself up for an easier time next year would be worth the tax on $10K.
The other thing you could do is just request a return of the excess contribution, pay tax on any earnings, and forget about your 2018 IRA.
Edited to strike anything related to having a 12/31 deadline - for what OP needs to do, that is wrong.