Frank Mittelbach wrote:
> In practical terms there is not much difference fully copying an instance
> declaration and changing a keyval or two and doing the same by something like
> \EditInstance. But there is a focus difference: if one copies the whole
> instance declaration to the document one doesn't see what modifications to the
> standard design have been done without comparing the instance declarations in
> detail whereas
> 
>    \EditInstance{sectioning}{chapter}{plain-head-A}
>           { font = optima }
> 
> directly tells that only the heading font gots modified. So it might be a good
> idea to provide such an interface extension for layer 1
> 
> frank

That to me seems like a good argument in favour of something like
\EditInstance. It is then mainly a question of whether we save the
keyval input or as I outlined "recover" the settings part of the code.
I'd probably favour the former, as it leaves open the possibility to
examine the current "state of play" of the settings. Also, you could
imagine doing the change by reading the existing keys into a property
list, doing the change and then re-saving, thus only applying each key once.
-- 
Joseph Wright