Yes, they can replace anyone (for any reason) up until first serve. Once the competition starts, then there will be no exchange/swap due to injury. (I'm not sure if covid is affected by this)
FIFA/IOC got approval (or was rumored) to just roster all of their alternates so there was no swapping for soccer... I was hoping IOC/FIVB might do the same
Chinese media reported that volleyball teams are allowed a 12 player roster +2 alternates hence Brazil Men's NT bringing Flavio/Gabriel and Turkey with Ayca/Asli. USA won't be able to bring all of SWP, Plummer, Dixon, Carlini, Courtney, and Tapp.
Also alternates won't be able to enter the Olympic Village.