If you have a thorough grounding and understanding of the PPL ground school syllabus, you will hit the ground running with the ATPL ground school.
The ATPL ground school material is not rocket science. The issue that a lot of people have problems getting to grips with, is the volume of material to get through in the time allowed. However, if you have a mature attitude and you plan your time accordingly and work hard at it for the duration of the ground school you won't have any problems at all.
Enjoy the whole experience, once you have finished and popped out the other end, its very rewarding.