I ended up folding over the object I need to convert and adding to a map, and then creating a new json object.
val conversionMap = Map("a" -> "b")
Json(
j.objectOrEmpty.toMap.foldLeft(Map.empty[JsonField, Json]) {
case (acc, (key, value)) =>
acc.updated(conversionMap.getOrElse(key, key), j.fieldOrNull(key))
}.toSeq: _*
)