Action to replace "[ ]" by "( )"

Use the offered help right there where the link points to ...
"How do I replace the string "xyz" in tag-fields or filenames?"
Actions
... and follow the instructions.

Example ...

$replace('square brackets [to] round brackets','[','(',']',')')

... will give ...
'square brackets (to) round brackets'

DD.20111115.1840.CET

Edit.
Example for the tag-field TITLE ...
Action: Format value
Field: TITLE
Formatstring: $replace(%TITLE%,'[','(',']',')')

Example for the pseudo tag-field _FILENAME ...
Action: Format value
Field: _FILENAME
Formatstring: $replace(%_FILENAME%,'[','(',']',')')

You can also use the action "Replace" to do so, but this will need two steps, collected in one group.

Begin Action Group Test_2011#20111116.Replace

Action #1
Actiontype 2: Replace
Field ______: TITLE
Original ___: [
Replace with: (

[_] Only as whole word [_] Case sensitive comparison

Action #2
Actiontype 2: Replace
Field ______: TITLE
Original ___: ]
Replace with: )

[_] Only as whole word [_] Case sensitive comparison

End Action Group Test_2011#20111116.Replace (2 Actions)

If you need to know how to create a new action, then see also ...
Actions

DD20111116.0740.CET