how to move contacts en mass from one iCloud account to another
From Mac Contacts..

File menu > Export > Contacts Archive

then

File Menu > Import



Or from iCloud…

Prefs 'cog'
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Export vCard - which will save a 'batch' vCard to your default DL location [then, irritatingly, immediately try to import it to your Mac Contacts list, so hit Cancel]

Swap iCloud accounts, then import vCard