Can't remove or fix "Unknown macro"

A number of old pages that were migrated to Confluence OnDemand contain macros that are no longer valid. They display in the page as, for example, Unknown macro: 'indent'

This macro has contents and the contents do not display.

{indent}
This is the content to be indented. It is not visible on the page.
{indent}

I can find no way to remove the indent macro, or change it to something else in the wysiwyg. I can find no way to recover the content wrapped in the macro, other than comparing older versions of the page which show the markup source — and manually copying each text block, on every page that has an "unknown macro" (thousands of our old pages)
This would be trivial to do in markup.