Please be clear---did you do anything to file the rejected 2018 return? Was it ever filed --either by e-file or by mailing it? If not, then you do not amend. Amending is a special process you only use if a return was filed, accepted and fully processed by the IRS.
If the return was never filed at all, then "amending" is not what you do. You finish preparing the return(s) and print, sign and mail the returns.
BUT--why do you think filing MFS would be better for you?
If you were legally married at the end of 2018 your filing choices are married filing jointly or married filing separately.
Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $24,000 (+$1300 for each spouse 65 or older) You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.
If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI)
https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separ...
If you really did file the joint return:
There is a rule that you cannot amend from filing jointly to filing separate returns after the filing deadline. You cannot change to Married filing separately for 2018 now.
If you still need to prepare and file your 2018 return(s):
Online preparation and e-filing for 2018is permanently closed. You have to download 2018 software to a full PC or Mac (not to a mobile device) and then print, sign and mail your return. If you already paid for your TurboTax software, you can contact customer support to push a download of the software to you.
To call TurboTax customer support
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
They are available from 5 a.m. to 5 p.m. Pacific time.
To finish a prior year return started in TT online:https://ttlc.intuit.com/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turb
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS and/or your state received the return.
NOTE: Since you will be using desktop software for this, you can try the returns both married filing jointly and married filing separately and see which is better for you.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
