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"Append" applies only to the Crypton Future Media Vocaloids. It is impossible to use the Appends without the original vocals, thus, the chances of them being retired is null right now.

While not necessarily appends, V2 Gumi has been replaced with V3 Gumi (Native) and V2 VY1 has been replaced with VY1v3. It's believed that MEIKO and KAITO's updates will require their original versions to be retired. The same may apply to Gakupo and V3 Gakupo as with VY2 and VY2v3.