According to the Soyuz MS-17 Wikipedia page, the spacecraft is planned to relocate from Rassvet nadir to Poisk zenith on 10 March 2021. This 3 weeks before the next Soyuz, MS-18 arrives.
From Wikipedia, Soyuz MS-18 is planned to dock Rassvet nadir, and will stay there for the duration of its stay.
Why won't Soyuz MS-17 dock to Poisk zenith from the start?
Or why can't Soyuz MS-18 dock to Poisk zenith, without relocating MS-17?
Note that this redocking and the Soyuz handover will happen with SpaceX Dragon Crew-2 docked on the US segment.