Maybe it has changed but it should be possible to hold an HK DL and NSW DL at the same time, I inquired at both the HK TD and the NSW RTA (before it changed name) and they would issue the local license instantly upon presentation of the other one, if you have an HKID / bank statement or similar with NSW address. They would give the other license back to you without marking it. The only restriction was that you had to be over 25 to get a full NSW DL otherwise you would be subject to the probationary tests.
I mean I need to change over if I live permanently back in Sydney. Not supposed to drive for over 3 months on IDP.
Not sure if redoing the vaccine is wise? Depends on how the boosters work, will there be a Pfizer booster different from the AZ booster?
IANAD
If same vaccine, overdosing can't be that bad. A vaccine conditions your immune cells to fight
SARS‑CoV‑2 (or parts thereof) when present, you're just asking your body to re-condition for the same fight repeatedly. It is just unnecessary but it can't be doing harm (as opposed to, overdosing some other meds like steroids or antibiotics).
UAE and HK are considering three-dose regimens for Sinovac (original dosing recommendation is two).
Different vaccine will have interaction concerns, but there's already trials underway (e.g. HK is either doing or referencing a study where we give Biontech (which we have, in surplus) to an arrival who already has one dose AZ (which we don't have))