Hi Abishek,
As far as i know there is no specific prerequisites for BCS. Check in UCMON and UCWB before and after upgrade. As a result of upgrade some of the objects used in BCS cube might become inactive. Make sure that all objects are active after upgrade. Also what is the version of SEM-BW component in 7.0? Direct upgrade of SEM 4.0 to SEM 6.34 is not possible.
Regards,
Raghu.