Is RTCA DO-260 Change 1 the same as DO-260A? I am a bit confused as to the history and the naming for 260 documents. This is the timeline I perceived.
From earliest to latest:

DO-260                  ( September 13, 2000 )
DO-260 Change 1         ( June      27, 2006 )
DO-260B                 ( December  2,  2009 )
DO-260B Corrigendum 1   ( December  13, 2011 )
DO-260B Corrigendum 1   ( December  13, 2011 ).
This one says it is supplemental to DO-260B for corrections in regards about typographical errors, etc.

Am I missing something? Please correct me if I'm wrong.

At the moment I assume DO-260A is the same thing as DO-260 Change 1 yes? Another is what are the major differences between DO-260B and DO-260B Corrigendum 1 ? Thank you in advance.

Sources:
RTCA store website,
Fourth Meeting of APIRG_ICAO
DO-260 change 1 is not DO-260A.

DO-260A released in April 2003
DO-260A change 1 in June 2006
DO-260A change 2 in December 2006

The releases marked as 'change' contain editorial changes, clarifications and corrections, but are effectively the same specification.
The alphabetical versioning is for subsequent releases of a new specification of the same system. In these releases the specification is changed, i.e. the behaviour of the system specified is different.