Keeping default optional argument when adding to command

What does LaTeX do when processing the definition of a command with an optional argument? Say we have

newcommand{test}[2][default]{Optional: #1; mandatory: #2.}

With a rather long sequence of steps that's not relevant for the discussion, the basic working is to do

deftest{@protected@testopttesttest{default}}
expandafterdefcsnamestringtestendcsname[#1]#2{Optional: #1; mandatory: #2.}

In the first line, test represents a single token with a backslash in its name, which can be obtained by csnamestringtestendcsname.

The macro @protected@testopt branches according to whether protect is relax or not and either calls @testopt (removing the else...fi part) or @x@protecttest (which removes everything up to fi and only leaves protecttest). The macro @testopt checks for a following [ and does the right thing.

So, if you want to patch the real replacement text of such a command you need to do

expandafterpatchcmdcsnamestringtestendcsname
  {<search>}
  {<replace>}
  {<success>}{<fail>}

It would be worse if you have

DeclareRobustCommand{test}[2][default]{Optional: #1; mandatory: #2.}

because in this case you'd need to do

expandafterpatchcmdcsnamestringtestspaceendcsname
  {<search>}
  {<replace>}
  {<success>}{<fail>}

For no optional argument, in the case of DeclareRobustCommand{test}[<n>]{...} the call should be

expandafterpatchcmdcsname test endcsname
  {<search>}
  {<replace>}
  {<success>}{<fail>}

This is the reason why I wrote the xpatch package, which is able to test how the command was defined to begin with and calls patchcmd with the right incantation. It also supports newrobustcmd from etoolbox and has built-in tools for patching internals of biblatex.

So you can simply do

usepackage{xpatch}

xpatchcmd{test}
  {<search>}
  {<replace>}
  {<success>}{<fail>}

and it will work in all cases.

What if you want to patch the default value for the optional argument? Even without knowing what the default value is?

documentclass{article}
usepackage{etoolbox}

makeatletter
defreplaceoptionalargument#1#2#3#4{%
  ifcsnamestring#1endcsname
    @replaceoptionalargument#1{#2}{#3}%
  else
    #4%
  fi
}
def@replaceoptionalargument#1#2#3{%
  edef#1{%
    unexpandedexpandafterexpandafterexpandafter{%
      expandafter@@replaceoptionalargument#1{#2}%
    }%
  }%
  #3%
}
def@@replaceoptionalargument#1#2#3#4#5{#1#2#3{#5}}
makeatother

newcommand{test}[2][default]{}

replaceoptionalargumenttest{new}{message{YES}}{message{NO}}

Addition about patching label

When you're trying to patch a command, you have to know in detail how it's defined. For instance, if you do showlabel in the preamble of your test document, you'll be presented the following information

> label=macro:
#1->@bsphack protected@write @auxout {}{string newlabel {#1}{{@currentlabel }{thepage }}}@esphack .

which is the standard LaTeX definition. Indeed, cleveref delays the redefinition at begin document. Good, let's issue showlabel after begin{document}, getting

> label=macro:
->@ifnextchar [label@optarg label@noarg .

which shows that label is not redefined like

renewcommand{label}[2][<default>]{...}

so your attempt will fail because label has no argument at all. Why does the developer do that way? Because there is no default value for the optional argument and different code has to be executed with or without the optional argument.