Federal - W4 Exemptions Question?

You should be able to safely change to married/1 and still not owe, but you might or might not get as big of a refund as you've been getting. That depends on what other itemized deductions you have. You'll also be able to take any real estate taxes, and either state and local taxes or sales tax, and any eligible charitable contributions. There are a number of other expenses that are allowable itemized deductions - download schedule A and its instructions at irs.gov to see what they are.