Regarding amending MFS to MFJ, see section 21.6.1.4.1 of the Internal Revenue Manual:
https://www.irs.gov/irm/part21/irm_21-006-001r
Also, from IRS Pub 17: https://www.irs.gov/publications/p17
Joint Return After Separate Returns
You can change your filing status from a separate return to a joint return by filing an amended return using Form 1040-X.
You can generally change to a joint return any time within 3 years from the due date of the separate return or returns. This doesn't include any extensions. A separate return includes a return filed by you or your spouse claiming married filing separately, single, or head of household filing status.
