According to documentation, since version 3.0 the default UUID representation is the `STANDARD` one.

Except for `UuidCodec` default constructor uses `JAVA_LEGACY` representation, making applications still store `LUUID` by default.

It is interesting to note that `ValueCodecProvider`, which is the default codec provider uses the default constructor for `UuidCodec`, making sure that `JAVA_LEGACY`/`LUUID` type is used by default.

I think the default constructor can be changed to `STANDARD` representation, as stated in documentation, and even in code:

// changed the default subtype to STANDARD since 3.0
if (encoderUuidRepresentation == UuidRepresentation.STANDARD) {
 writer.writeBinaryData(new BsonBinary(BsonBinarySubType.UUID_STANDARD, binaryData));
} else {
 writer.writeBinaryData(new BsonBinary(BsonBinarySubType.UUID_LEGACY, binaryData));
}

Environment variables could be used to restore legacy behaviour withour resorting to mess with code.