William Pence wrote:

One of the proposed changes, however, is to require that the
PCOUNT and GCOUNT keywords must immediately follow the last NAXISn
keyword in all conforming extensions,

It seems to me that if this or any future revision to the standard is
going to attempt to require newly defined usage that this MUST be
paired with a version mechanism. Otherwise "once FITS always FITS"
is violated big time. Requiring a feature (a demand) is not the same
as deprecating its converse (a suggestion). Rather, a feature that
isn't already required can only in the future be required against
some specially labeled subset of FITS files.

I agree with Preben that there are also problems with introducing
versioning, but otherwise conformance is unenforceable. The
suggested new wording of 3.7:

‘Existing FITS files that conformed to the latest
version of the standard at the time the files were
created are expressly exempt from any new
requirements imposed by subsequent versions
of the standard’.

can't serve as a substitute for an explicit version since A) not all
files have DATE keywords, so how would you know, and B) it will take
several years - perhaps never - for all prior software to adopt the
new standards. Rather, projects wanting to assert compliance would
include a recognizable FITSVER (for example) keyword. Files missing
this tag would by default only claim compliance with the original
standard.

In a real sense, the standard as revised is really a brand new
standard, subclassed out of FITS.

- Rob