If you have already registered for CA-CPT on or before 30th June 2017, then only you are eligible to appear for CA-CPT December 2017. Another condition is that you should have passed 12th Standard by now, for you to appear for CA-CPT December 2017. If any one of these conditions are not satisfied, then you are not eligible to apply for CA-CPT exam scheduled in December 2017.
If you are eligible, and appear for CA-CPT in December 2017, then the old syllabus only applies.
The new syllabus is applicable for students registering on or after 1st July 2017. They will be writing their CA-Foundation (which is the new nomenclature for the previous CA-CPT) exams in May and November Vs June and December for CA-CPT.
The changes have come to effect. But if you have registered for CA-CPT on or before 30th June 2017, then you will have four more attempts under the old syllabus scheduled in December 2017, June 2018, December 2018 and June 2019. Till such time, you will remain in the old syllabus. But the Institute may shortly come out with an option to shift to new syllabus during this intervening period too, at the option of the student.
Wishing you the very best of your luck and efforts to clear CA-CPT in the ensuing December 2017 exam, if you are otherwise eligible to appear for the same.