CX-310-065 is the full
SCJP 6 exam, CX-310-066 is the upgrade SCJP 6 exam (see
here).
The upgrade exam is meant for people like you, who already are SCJP certified for an older version, and who want to upgrade to SCJP 6. The content of the full and the upgrade exams is the same (the same topics are covered), and the certificate for either exam is the same. The upgrade exam has less questions (48 instead of 72 questions).
So, do the upgrade exam CX-310-066 if you're already SCJP 1.4 certified.