If the City moves to districts for the 2020 election will the City then have to redraw the districts in 2021 after the 2020 census data?

Yes.The City’s next election is 2020, and the lines would have to be reconsidered upon the release of the Census the following year. See Elec. Code § 21620 which states “After the initial establishment of the districts, the districts shall continue to be as nearly equal in population as may be according to the latest federal decennial census or, if authorized by the charter of the city, according to the federal mid-decade census.”