Registration in new course of CA final

Yes. You can register/convert to new syllabus anytime. The group cleared or paper exempted shall be carried forward. But once you apply for exam in old syllabus(for which time has expired) you must give your exam in old syllabus only.
In course training called AICTSS which is a combination of gmcs1, gmcs2 and adv.itt. So if you have done gmcs1 and you have the certificate, then only the remaining part of AICTSS shall be done. For more details regarding this, you can check out the BoS portal in students section(FAQ's).