I believe that most users would be willing to migrate from their
current 2.x version to a new version (say 2.6.5, to include the magic
"5") which had
(1) a DOS-like interface identical to 2.6.3in, but with support for
the additional algorithms of 5.x (SHA1 and DSA, EG, CAST5, 3DES);
(2) with key generation extended by optional additional parameters
to specify these algorithms (somewhat like PGP 5.5's choices);
(3) with defaults for these additional parameters set in pgp.ini for
existing programs which shell to PGP to generate keys.
A fourth point (which you've already touched on with your mention of people
running PGP on pocket calculators and the like) is that it'd be a good idea to
finally split the key management off into a seperate program, especially with
the extra bloat added by the new algorithms and key packets. About 99% of the
time you never need to use the key management features, and you could probably
halve the size of the program by moving the code into a seperate executable.
If people really want to be able to run it under a single name, you could add
a wrapper which invokes pgpcrypt or pgpkey depending on whether any of the
args begin with -k.
Peter.