Couples who have been in a Civil Union may convert their civil union to a marriage during the grace period. They have the option to set their marriage to be effective as of the date of their original civil union.
Such couples can amend their federal income tax returns for 2011, 2012, & 2013 to claim married-filing-jointly status and possibly receive a refund or lower a tax bill.
If you elected coverage for your partner under employer-provided health insurance while in a civil union, your employer included the cost of coverage on your W-2. The cost was income on your federal income tax return. Converting the civil union to a marriage allows you to amend your returns (2011, 2012, 2013) to exclude that income - possibly reducing tax due or obtaining a refund.