Can assign a new domain to a site in a multisite?

Go to the Network Admin screen of the site, then Sites, then Edit the site. Put the new URL in the various locations.

Added

When you Edit the site, select the Settings tab. That will allow you to change the Home and Site URL values (which writes to the wp-options for that site). There may also be several other places to change the URL for other options settings by plugins or themes.

You may need to do a Search/Replace (I like the ‘Better Search and Replace’ plugin) to change URLs for media for that site. And a backup of the databases is always a good idea before changing.