Also a requirement in in NZ FIRs: In the domestic FIR, it is for IFR flights without an assigned code, in the Oceanic FIR it is for all VFR and IFR flights without assigned codes ( eg when going NFFN-NZAA, or similar).
It seems to be only in US FIRs (eg changing from domestic to Oakland Oceanic, for example) that the controlling agency requires a change from the previously assigned code to squawk 2000. Perhaps to do with ADIZ or other airspace control?