We can not change the document type (MEREQ_TOPLINE-BSART) of PR which created from MRP by ME52N, this is because the following hard codings:

Main Program     SAPLMEREQ

Source code of   LMEREQF56

       IF l_context->header_new->trtyp NE hin AND

          l_context->header_new->header-estkz NE ‘R’.         <<<<<<<<<<<<<<<<<<<

         l_context->header_new->header-bsart =

             l_context->header_before_checks->header-bsart.

       ENDIF.

Like above, the system checks if the PR was created manually (R). If not, then system will change the document type back to the original one.