The NCM admin manual says (p. 17)
Note: Because SolarWinds NCM 7.x shares common components with all Orion Platform products, the process of installing and configuring SolarWinds NCM involves shutting down and restarting all Orion services. Choose a time to install SolarWinds NCM 7.x when your IT operation can tolerate a short period of downtime for Orion Platform products.
However, earlier (on p. 14) for Guidelines it says
Migrating all data (4-6 Hours)
So is NPM only down briefly and the NCM upgrade takes potentially 4-6 hours, or... ?