The Injured Spouse form is used to prevent one spouse from being penalized by the other spouse's past debt. You are not penalized if you don't use it, but you are not protected either. You can amend to file jointly and include the Form 8379 along with it. Amendments are not yet available to be filed.
Once you've filed your tax return as Married Filing Separate (MFS), you have three years from the original deadline to make the change to your tax return.
