You can change the category of an already-posted work to "manga" on the desktop version of pixiv.
It cannot be done from the mobile version or from the app.
The procedures are as follows:
- From your Dashboard, go to "Works" > "Illustrations" and then click the pen mark next to the title and thumbnail of the work you want to change the category for.
- In the "Type of Work" category, select "Manga" and then click "Save" at the bottom of the page to change the category of the work.