Wasn't able to get wife's SS# until December Last Year. Can I now change the 2019 fed return to MFJ?

If you filed as MFS, you can amend your return to MFJ.

If you are also adding kids, you will not be able to claim EIC or CTC as the laws changed a few years ago to disallow claiming EIC.

The Protecting Americans from Tax Hikes or PATH Act made several changes to the tax law, including preventing retroactive claims of the EITC and CTC by amending a return or filing an original return for any earlier year in which the individual or anyone listed on the return did not have a SSN valid for employment. You can't claim EITC or CTC unless the Social Security number for you, your spouse (if married filing a joint return) or a qualifying child is issued before the due date of the return including any valid extensions.