The Architect of KM says there is no search and replace within a macro.
There might be people who have accomplished this using XML and uploaded solutions to the Macros page on this website. It's probably not 100% reliable because strings can occur in all kinds of strange places in actions.
You can search for strings in the editor, but there's no replace.
Perhaps the lesson here is never hardcode string constants, at least not repeatedly. Always use variables so you have to change something only once.