You can't change from joint to married filing separately, but you should be able to change from married to single. However, the IRS may have a problem with that since, in order to file as married filing jointly, your spouse has to agree to a joint filing and "sign" the return. The fact that you can file electronically without really getting your spouse to sign doesn't change that basic rule. The IRS may contact you and ask for proof of when the divorce was final. (But don't include any extra un-aked for paperwork when you file, just give the explanation on the 1040-X.
How did she file? Because any change you make will affect her. If she filed jointly with you, she will also have to file an amended single return. If she had no income, but you paid alimony after the date the divorce was final, you can deduct that but then she will have to file and report it. OR if she filed separately AND you filed jointly, the IRS should not have allowed that.
Amending instructions are below. Don't forget to also amend your state tax return.
Amend https://ttlc.intuit.com/replies/3288565
