Hello
The nightmare is going on... this point has been discussed a year ago (Re: Comapatability between EHP7 and SAP Netweaver BI 7.0 EHP1) and the conclusion was that you only need to upgrade HUB satellite systems... and a BW system is not an HUB.
The only possible issue would be if the java stack of the BW system is used directly by the ECC system.
Did not find any information on the necessity to upgrade the CRM, so I think you should keep it as it is.
This are even getting worst because sometime sap support (OSS) are not able to provide clear answers (Interoperability btw ERP 6.0 EhP7 and SAP NW/BW 7.0)
Best regards
Strategy beyond SAP Business Suite 7 Innovations 2011 - Landscape Governance - SCN Wiki
1388258 - Version Interoperability within the SAP Business Suite
In order to use the new JAVA EE 5 capabilities and benefit from the improvements of SAP NW 7.3 the version interoperability range has been moved upwards for SAP Business Suite core applications with Release to Customer (RTC) from 2013 onwards.
This means that SAP NetWeaver Systems 7.30 or higher are required if SAP Business Suite applications are based on or operating with those central Systems (Enterprise Portal, Business Warehouse, Process Integration).
Customers wishing to utilize SAP Business Suite applications with RTC from 2013 onwards, that require one of these central SAP NetWeaver Systems, will have to upgrade their central NetWeaver Systems to 7.30 or higher before upgrading e.g. to SAP Enhancement Package 7 for SAP ERP 6.0.