There is a defined lapl to ppl route which covers the differences in the syllabus. I undertook that this year, 15 hours min, 10 of which must be at an ATO, 6 dual and 4 solo I think. I had to do a GST and the 9 ground exams but I had went nppl > lapl > ppl which is slightly different.
So if you did complete the lapl syllabus then decide to change, it isn't too much of a hassle.