EHP5

We are currently using old JAVA/Web Dynpro based ESS. My client is interested in upgrading just the Leave Request application to new EHP5 ABAP/Web Dynpro based solution and keep rest of the functionalities in ESS like Pesonal Info, Benefits etc in old java/web dynpro. I would like to know if this is possible? If yes, are there any down sides for using partial new EHP5 functionalities?

Thank you,

Bobby