Is RTCA DO-260 Change 1 the same as DO-260A?
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.