Hi Stephen,
I think if you need it, you can do it manually in database. You can investigate how different page types are saved in database and update corresponding fields.
I've done this conversion only once, so you may do it only at your own risk. Do backup before)
Pages -> Content only pages
declare @ClassID int = 0 --your class id
update CMS_Class set ClassIsContentOnly = 1 where CLassID = @ClassID
update CMS_Tree set NodeTemplateID = NULL where NodeClassID = @ClassID
update CMS_Document set DocumentPageTemplateID = NULL, DocumentContent = '', DocumentWebParts = ''
where DocumentNodeID in (
select NodeID from CMS_Tree where NodeClassID = @ClassID
)