It seems that changing content model from eg. "wikitext" to "Page: page" for pages in Page namespace no longer works.
While attempting the change I get an empty page (with no HTML at all) and no evidence about the attempt in available logs (the page content model remains unchanged).
Tested on plwikisource and sourceswiki. The MediaWIki behaviour is the same if the page already contains a ProofreadPage header/footer as if it does not contain any.
This used to work at least in enwikisource, as the following log shows:
so I assume it is regression of MediaWiki functionality introduced in the meantime.