Hi Karthik,
Upgrade of BW 3.1 (and also BW 3.5 a.k.a. NewWeaver 2004) is not directly supported. The mimimum release is 7.0 (SP 14 or higher), 7.0 with EhP 1 or 7.0 with EhP 2. I found this information in the Upgrade Master Guide SAP NetWeaver 7.3 incl. Enhancement Package 1 (document version 1.13) paragraph 2.3. You can find this document here.
An approach could be to do the upgrade in different steps, e.g. from 3.1 to 7.0 and from 7.0 to 7.3. I advise to ask for support from SAP to validate the best upgrade scenario.
Best regards,
Sander