Is it possible to change the doctype of a bizdoc when it’s already persisted? It works while it’s still in memory by simply doing a wm.tn.doctype:view and mapping the output to the bizdoc/DocType.

But when I changed my process to persist the bizdoc earlier, this doesn’t work any longer.

I also tried wm.tn.doc:updateSystemAttributes but it did not work (documentation says so).

No that would not be possible, as once persisted the bizdoc will keep the inherent settings of webMethods and gets stored in to its internal db. The db would not accept a changed bizdoc. In the pipeline you can change whatever you want, but what i would suggest is that you make a copy of the bizdoc document and manipulate it to your use instead of modifying the original bizdoc.