recommended approach to move towards WDA based ESS/MSS but as earlier its a project If you want to stick for WD JAVA for a while till you configure WDA ESS/MSS please deploy new BPs to run on NW 7.3 ie For NW 7.3 you need to use the ESS 633, MSS 630 & PCUIGP 633 services. These are the JAVA services in ESS written especially for NW7.3. Can you share Your version, as it is incompabtible and it wont work in above NW 7.3 as we have new BPs which needs to be deployed to run on 7.3
please check above
314568 SAP GUI for HTML functionality / Limitations / Sp.
1007341 - SAP ERP 2004/2005 - supported ESS/MSS system landscapes
1854758 - Standard Mode support for WebDynpro Java applications
