Thats a good question I reckon.
I think you will be allright with the charts you have. Very Unlikely you will be caught out by one day. Can you view the amendments to the charts online and note them on your charts? If you are in Perth I doubt Ma and Pa are going to notice...
The interesting parts of the question are
How many (significant) changes are made from edition to edition?
Can you just note the changes made to the last chart from the list of amendments in the new chart?
Will CASA questions rely on some of the subtleties in amendments to charts?
I have my ATPL Nav and Law exams soon and I do not want to waste upwards of 300 clams on pubs of no other use to me.