You can't rollover a Roth 401k to a pre-tax 401k. You could stop making contributions to a Roth 401k and start making new contribution to a pre-tax 401k, but you can't move existing funds in that way. Regarding impact on your paycheck, it depends on your income, filing status and dependents. A rough estimate is that if you can contribute 12% or 22% more, plus an amount for state taxes.
For example, suppose you are in the 12% income tax bracket and pay 5% tax in your state. If you contribute $100 to a Roth 401k, it will reduce your paycheck by $100, but if you contribute $100 to a pre-tax 401k, it will reduce your paycheck by $83. So you could increase your contribution to about $120 to reduce your paycheck by $100.
