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A.  Codes assigned by ISO 639-1 that do not conflict with
    existing two-letter primary language subtags and that have
    no corresponding three-letter primary defined in the
    registry are entered into the IANA registry as new records
    of type 'language'.  Note that languages given an ISO 639-1
    code cannot be given extended language subtags, even if
    encompassed by a macrolanguage.

And from RFC 5645, Section 2.2 ("New Language Subtags"):

   For each language in [ISO639-3] that was not already represented by a
   language subtag in the Language Subtag Registry, a new language
   subtag was added to the registry, using the [ISO639-3] code element
   as the value for the Subtag field and using each of the non-inverted
   [ISO639-3] names as a separate Description field.  The [ISO639-3]
   reference name is represented by the first Description field.

   If the language was encompassed by one of the [ISO639-3]
   macrolanguages 'ar' (Arabic), 'kok' (Konkani), 'ms' (Malay), 'sw'
   (Swahili), 'uz' (Uzbek), or 'zh' (Chinese), as determined by
   [iso-639-3-macrolanguages_20090120], an extended language subtag was
   also added, with the primary language subtag of the macrolanguage as
   the value for the Prefix field.

There is no similar wording in Section 2.3 ("Modified Language 
Subtags"), by design.  In the (many times repeated) production of the 
RFC 5645 Registry, only newly created language subtags were considered 
as candidates for extlangs.  Existing language subtags were not examined 
as part of this process.

--
Doug Ewell  |  Thornton, Colorado, USA  |  http://www.ewellic.org
RFC 5645, 4645, UTN #14  |  ietf-languages @ http://is.gd/2kf0s ­