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.