minecraft-server-resource-pack/resourcepack/assets/minecraft/items/oak_sapling.json

{
  "model": {
    "type": "select",
    "property": "component",
    "component": "minecraft:custom_name",
    "fallback": {
      "type": "minecraft:model",
      "model": "minecraft:item/oak_sapling"
    },
    "cases": [
      { "when": "christmas", "model": { "type": "model", "model": "lz:festive/christmas_tree" } }
    ]
  }
}