This is actually an interesting idea that I hadn't thought of before. I think your plan could work out if you plan to retire in a lower tax bracket than you're in now and you are eligible to make pre-tax traditional IRA contributions and you have no other source of funds to contribute to your IRA.
Let's look at some numbers.
Suppose you contributed $10,000 to a Roth IRA in the past. These funds are freely withdrawable at any time. Suppose you expect your investments to double in value between now and when you withdraw. Suppose you're in the 22% bracket now and will be in the 10% bracket during retirement.
Scenario 1: Leave the money where it is
Your $10,000 grows to $20,000 by the time you want to withdraw it, and you get to keep the whole amount because it's in a Roth account. Simple.
Scenario 2: Withdraw the money and contribute it to a traditional IRA
You take the $10,000 out of your Roth IRA, contribute it to a traditional IRA. You save $2,200 on your taxes. You put $1,500 of that into your traditional IRA (since the IRA limit is $5,500 in 2018 and $6,000 in 2019), saving another $330 on your taxes. That leaves you with $1,030 of tax savings outside of your IRA. You invest that in mutual funds in your taxable account.
Retirement comes around. Your investments have doubled, leaving you with $23,000 in your traditional IRA and $2,060 (minus a bit for dividend taxes in intervening years) in your taxable account. You pay 10% income tax on the traditional IRA withdrawal and 0% capital gains tax on your taxable account, leaving you with $20,700 from the IRA and $2,060 from the taxable account.
This is more than you would have had if you leave the money in your Roth IRA. However as I see it the flaw with this plan is the assumption that you're already in a high tax bracket but somehow have no funds leftover to contribute to an IRA without raiding your Roth account. In the very narrow set of circumstances where this is the case the math does seem to support making this maneuver, but I'd challenge you to find a way to scrape some more cash together before April to make this contribution in another way.