At NASA "Shall" Now = "Must" (or Does "Must " = "Shall"?). Got That? [Revision]

NASA Internal Memo: Federal Plain Language Guidelines – IMMEDIATE ATTENTION REQUIRED
“The subject guidelines endorses doing away with the term “shall” to mandate requirements and using the term “must” instead. These guidelines were reviewed by the Office of the General Counsel who determined that these guidelines are arguably not mandatory for most of what we do, but prudent to implement across- the- board and use “must” instead of “shall.” Therefore, NPR 1400.1, NASA Directives and Charters Procedural Requirements, will be adjusted to include the term “must,” to denote mandatory action prior to approval. This requirement will be effective when NPR 1400.1 is approved. Please inform your directives reviewers and writers.”
Keith’s 8 June update: Nannette Jennings at NASA just posted this in the NASAwatch comments section:
NASA will continue to use the term “shall.” The following updated e-mail was distributed to the Agency’s Directives Managers on June 1: The following provides an update to the subject e-mail sent May 24, 2011: As stated in the initial e-mail, the Office of the General Counsel (OGC) reviewed the subject guidelines and determined that the guidelines are arguably not mandatory for most of what we do, but prudent to implement across- the- board and use “must” instead of “shall.” OGC was asked to review these guidelines again and their view is that using term “must” instead of “shall” does not apply to internal directives, but can be implemented if the Agency decides to. Further, the Office of Management and Budget was consulted and they confirmed that using the term “must” IS NOT a requirement. It’s up to the agencies to use the term. Therefore, the Agency WILL continue to use the term “shall” to denote mandatory requirements.
Speaking for myself, I’d be more confused about official “shall” and “must” usage guidelines at NASA than I was at the onset – now that OGC has clarified the matter, that is. Apparently at NASA, there is no “must” or “shall” involved with the use of the words “must” or “shall”…
Confused? Consult the official “Federal Plain Language Guidelines“ at plainlanguage.gov and you must shall understand, I guess.