A Roth conversion is disregarded with regard to the Retirement Savings Contributions credit. You'll just pay income tax on the conversion, no repayment of the credit.
If less than 85% of your 2019 Social Security income will otherwise be taxable, the extra income from the Roth conversion will likely cause more of your Social Security income to become taxable, having a multiplying effect on your tax rate opposite of what has occurred on your 2018 tax return, so be careful about making any assumptions about your marginal tax rate. But yes, there are situations where shifting taxable income to the following year can produce a net savings.
You need to prepare a simulated 2019 tax return to be sure. You can use the What-If Worksheet in forms mode of the CD/download version of TurboTax to do this.
