There is no penalty for changing your W-4, that is your choice.
The issue is if you will both have enough taxes withheld when you file as Married Filing Jointly for 2017 so that your total tax payments will be equal to or greater than your total tax liabilities.
You can use the TurboTax W-4 withholding calculator - <a rel="nofollow" target="_blank" href="
https://turbotax.intuit.com/tax-tools/calculators/w4/">https://turbotax.intuit.com/tax-tools/calcula...>
The IRS W-4 withholding calculator - <a rel="nofollow" target="_blank" href="
https://www.irs.gov/individuals/irs-withholding-calculator">https://www.irs.gov/individuals/irs-with...>
You can also use the TurboTax calculator TaxCaster to get an estimate of any tax refund or taxes owed when filing your return as MFJ with your incomes and current tax withholding - <a rel="nofollow" target="_blank" href="
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/">https://turbotax.intuit.com/tax-tools/...>
