If you want to pay the tax now, you can make an estimated tax payment for 2020. That's completely separate from your 2019 tax return. You can't tell now exactly how much the tax will be, but you can estimate it. (That's why it's called "estimated" tax.) Any difference between your estimated payment and the actual tax will be reconciled when you report the conversion on your 2020 tax return. For a transaction on July 13 the estimated tax is due September 15. Here are a couple of FAQs about estimated tax.
What are federal estimated taxes?
How do I make estimated tax payments?
