per @cmgchess -

This can all be done in two steps:

• (starting point)

{
  _id: 1,
  categories: [123, 234],
  // other fields
}

• $lookup (without unwinding)

{
  "_id": 1,
  "categories": [
    {
      "_id": 123,
      "value": "Category name"
    },
    {
      "_id": 234,
      "value": "Other category name"
    }
  ],
  // other fields
}

• $set using $map

// operation:
{
  $set: {
    categories: {
      $map: {
        input: "$categories",
        as: "category",
        in: "$$category.value"
      }
    }
  }
}

// result:
{
  "_id": 1,
  "categories": [
    "Category name",
    "Other category name"
  ],
  // other fields
}

paydirt. :)