Taxes would hit when you pay your 2019 income taxes.
Tax advantage is tax free growth on the Roth - so when you take out the money post-retirement it is tax-free.
Your 2018 tax returns should give you an idea how much you might pay in taxes if you move it into a Roth. There may be a better explanation, but (made up numbers):
If the tax liability for $52K-$62K is $10K, and you at 52K, that $8K rollover falls into that tax bucket and doesn’t cost you anymore in taxes.