It is very easy to toggle between find and replace functionality.
When you perform search and replace in a file, pressing Control+F;Alt+F3 shows the search pane. Pressing Control+R adds field, where you can type the replace string.
While in the Find in Path dialog, you can switch to replace by pressing Control+Shift+R. Same way, press Control+Shift+F to hide the Replace with field, and switch to mere search.