You are correct that there are additional steps for incorporating the updated occupation codes in the 2020 data to the harmonized occupation measures. Additionally, as noted in this forum exchange, we are seeing some unexpected assignments for certain occupations. The IPUMS CPS team is hoping for a bit more information from BLS on editing procedures that they implemented for the 2020 data before moving ahead with creating the harmonized occupation measures in 2020 BMS data.
The Census Bureau occupation and industry codeslists and crosswalks page has a crosswalk between the 2018 occupation codes (used in 2020 CPS data) and 2010 occupation codes (used in 2011-2019 CPS data). This should help you assign the occupation codes in the 2020 BMS data into the 2010 scheme (which is what OCC2010 will do eventually). Additionally, this working paper outlines the process IPUMS uses to leverage the crosswalks between coding schemes and make decisions about occupation codes with an unclear assignment to the adjacent scheme.