Hi
From NW 7.4 (CRM 7.0 EHP3) onwards SAP will not support dual stack. In CRM system you have to install CRM ABAP & JAVA in different SID's. so better you can do the fresh installation, keep both stack on same version
Kindly refer the CRM installation master guide
http://service.sap.com/~sapidb/011000358700000726052013E/crm_master_guide_ehp3.pdf
BR
SS