In https://gerrit.wikimedia.org/r/207353 @Reedy says we should have a maintenance script to change a page's content model. Or presumably change the content model of a list of pages, I guess?
Change 221783 had a related patch set uploaded (by Lewis Cawte):
[WIP] Add a script to change the content model of a page
This is not simple at all (dozens of code lines), hence removing the good first bug tag.