FAA ATP
The rule change will mean a check ride to convert the ATP to CASA equivalent.
I'd get it done before the rule change... At the moment it's a relatively painless exercise and the guys and gals at CLARC are actually really helpful.
You need to write the same exams whether you convert the cpl first and the ATP later, but I think you need to do an extra flight test by converting cpl first.
Are you flying for an Ozzie company on a validation at the moment? If you're in a CAR217 organisation then your IR and CPL flight test can be done in-house as part of a prof check. Same will apply to the ATPL flight test when it comes into force.
PM me if you want more specific info.