Something like this...though my syntax may not be exactly right...
Then you can log into the new account and share the new calendar again...or from CLI
#### as zimbra user
prov> sm(selectMailbox) ACCOUNT
mbox ACCOUNT> gru(getRestURL) -o /tmp/data_file.ics /Calendar
mbox ACCOUNT> exit
prov> sm ACCOUNT2
#### if calendar is not created in the new account yet
mbox ACCOUNT2> cf(createFolder) -V appointment /CALENDAR_NAME
mbox ACCOUNT2> pru(postRestURL) /CALENDAR_NAME /tmp/data_file.ics
Users will have to remove their shares to the old calendar, or you can do that for them from the CLI also...
cm(createMountpoint) /CAL_NAME ACCOUNT2(calendar owner) /REMOTE_CAL_PATH
Hope that helps...