Welcome to the Ranch.
If you are attempting OCPJP6 then the exam is 'Java SE 6 Programmer Certified Professional Exam' (exam 1Z0-851).
The upgrade exam is for people who already have OCPJP in an earlier version.
As for your Q2, which version to take depends on your experience. V7 needs 2 exams to get OCPJP while V6 only needs 1 exam. V7 exams are divided into 2 levels (associate
OCAJP and professional OCPJP). The OCAJP7 exam is for beginners covering the fundamentals like syntax, OOP concepts, flow control. The OCPJP7 covers more advanced stuff like
JDBC, NIO, threading/concurrency, collection API, some design
patterns.
On the contrary, the OCPJP6 exam covers materials what every Java programmer should know. Topics range from fundamentals and OOP concepts to collection API and threading/concurrency. Simply put, if you are to compare the OCPJP6 and OCPJP7 exams, the V6
does not cover JDBC, NIO, localization and design patterns.