just now we have SP 9 . we are planning to load SP 13 . My query is will it overwrite all the standard sessions ? or add only new session . why i am asking this because we have customised many standard sessions also

It will not overwrite your customizations because new objects are put into the Update Standard VRC and your customizations should be one level higher. But most likely new objects will contain modified code and functionality. This means you have to review (and modify) all your customizations anyway. It is the pain with lots of customizations in general :eek: .

Martin