Symptoms

1. When upgrading Confluence, the page does not convert from from wiki markup to XHTML storage format
2. Running the Force upgrade process does not convert the page

Workaround

1. Edit the page in question 

2. Select the entirety of the page and copy the contents into your clipboard

3. Click Insert > Markup on the editor toolbar
4. Paste the wiki markup into the dialog and click insert. This will automatically convert the markup into XHTML.
5. Save the page