appears to have no flexibility to do this and there are no version
fields in the frame structure so it can't be made backwards compatible.
The design ought to have had this built in, but seems not to have
considered doing it.

The exact same stupidity has long crippled the advancement of the
AX.25 protocol, and the APRS protocol, and nearly every other *amateur*
designed protocol.

Worse, AX.25 was coded into law (in the USA, and elsewhere), thus
preventing any other air protocols from even having a chance because
they could not be used, or used unattended, such as a BBS.

D*Star apparently has the same problem - it's a lot like commercial P25
(or MotoTRBO), but it's a lot different too, and there's really no way
to fix that.

And how about all those surplus TETRA radios? Yah, the USA can't currently
use TETRA for commercial/industrial radio, but the rest of the world does
and there's nothing stopping USA hams from doing so.

I sure would like to see someone make an off-the-shelf digital voice ham
radio that made use of openly-available codecs, especially if they were
flashable.

AX.25's other remaining use, that of carrying tcp/ip, has also got a
real problem: the minimum IPv6 packet is bigger than the maximum AX.25
packet. Oops.
- Brian