BatchEdit plugin
This plugin does wiki-wide find and replace for page text. It is located on the Admin page under Additional Plugins > BatchEdit.
Pressing that button opens up a straightforward find/replace page. Searching within a particular wiki namespace speeds it up considerably. The Search for and Replace with boxes work as one would expect: one can use plain text or regular expressions. Both boxes can be used in single- or multiline mode. The Edit summary box is used as in the usual page editor.
Press preview to check the replacements made. One can then check the box(es) for the desired places in which to apply the change. The top-level checkbox selects them everywhere, all instances on each page can be selected or each instance individually. Pressing Apply then performs the replacement.
There is a limit to how many replacements can be performed per search. The present editor has not yet found a setting for increasing or overriding this, which is probably for the best if sometimes annoying.