I converted from multi member llc to a single member llc. Do I still issue K1 to my old partner and myself or do a 1099-NEC to her now and file Schedule C for myself?

A combination of both situations will apply in the creation of the "tax reporting documents" for the multi- and single-member LLC's members for their individual income tax return(s). The LLC could have two partial tax years if it is operated part of the tax year as a multi-member LLC and part of the tax year as a single member LLC.

Schedule K1s will need to be issued to both multi-member LLC members by the LLC to report the income and expenses for the multi-member LLC's tax year before its termination (as long as it did not previously make the election to be taxed as a corporation). A multi-member LLC, in this case is taxed "as a partnership" on Form 1065 generating k-1s for each member. The Schedule K-1s should be included on their individual income tax returns on page 2 of Schedule E.

How to handle multi-member LLC termination to a single member LLC entity.

As long as the single-member LLC owner has not made the election with IRS to be taxed as a corporation, then Schedule C of the individual income tax return of the single member LLC owner will be used to report the income and expenses for the portion of the tax year after its creation. Form 1099-NEC is not required to be used in this situation because as a single owner of the newly created LLC, the owner is considered a sole-proprietor for income tax reporting purposes.

Yes, you are filing two separate short year business returns. The change from multi-member LLC (taxed as a partnership) to single member LLC (taxed as a sole proprietorship) occurring on March 27 causes the multi-member LLC to file a final short tax year return for the period from January 1, 2020 - March 27, 2020 issuing Schedule K-1s carrying to the Sch E page 2 of the taxpayers' Form 1040. The new single member LLC's tax information will be reported on Schedule C on the owner's personal income tax return and would cover the period of time from March 28, 2020 through December 31, 2020.

The full year income and expenses of the business operations will need to be broken down between the two periods by date, because they are changing the entity to two separate businesses and filing two separate entity business returns for the year, one on Form 1065 (multi-member LLC taxed as partnership) and one on Schedule C as sole proprietorship.