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Suppose I create a command with xparse
NewDocumentCommand{\myCommand}{ m O{} }{ #1 and #2 }
Could I later in my document change the default value of arg two? Maybe something along these lines?
\def\@myCommand@args@II{New Default}
(II denotes the second argument)
So that later on, \myCommand{Hello} produces "Hello and New Default"
You might do
\NewDocumentCommand{\myCommand}{ m O{\DefaultArgForMyCommand} }{ #1 and #2 }
\NewDocumentCommand{\DefaultArgForMyCommand}{}{}% initially empty
and then do
\RenewDocumentCommand{\DefaultArgForMyCommand}{}{whatever}
when you want that the optional argument changes its default value.
However, I can't recommend doing this unless you know without any possible doubt what's the value of \DefaultArgForMyCommand at any given point (I'm not sure you can).
\myCommandbe component of a moving argument? Will\myCommandbe, e.g., component of a sectioning-command which ends up in the table of contents and in the bookmarks and in page-headers as well? "moving arguments" in LaTeX are macro-arguments which are carried out at several points of time during compilation so that at one point in time the definition of a macro-token which is component of such a "moving argument" might differ from the definition which the macro-token in question has at another point in time although at each point in time you wish to obtain the same textual phrase...