Hi Optima,
I'm currently going through the same process for my 14.44 which has existing ATM of 1930, I'm looking for a similar increase to you. I am now at the engineering stage and still have a few things to work out. Based on my experience I doubt you can just change the springs (sorry) you will also need to look at axle, coupling, rims/tyres ratings and possibly chassis/drawbar. You will also need a breakaway brake system when ATM over 2000kg. I found all components were reasonably closely rated to the original GTM/ATM, except tyres which were 105 (925kg), very close to the limit (thanks Jayco).
Another approach might be to look at increasing the towball loading so van GTM (ATM - ball weight) is under 2000kg and might avoid changing springs etc. You will need to consider if your vehicle will support this (and allow for the reduced vehicle payload). For example if you have a towball mass of 300kg, the ATM could be 2300kg with the van itself (GTM) only 2000kg. I hope that makes sense, I've been doing a lot of research on this lately.
Depending on the state you will have different requirements/approval to keep it legal, insured etc. in Vic we need an engineering certificate for any changes to the compliance plate details.
Regarding tyres/wheels, there are a tonne of options... Be careful to take the rim offset into account as well as capacity otherwise your new wheels might not fit. Common tyre sizes in 16" appear to be 245/70R16 and 225/75R16. If your diameter increase is too much it might downrate your brakes, but assuming you have the 12"*Try posting a bit more so your post isn't boring*" Al-ko should be ok. Apparently 265/70R16 used to be a factory option but these could rub the guards (depending on how accurate the guard placement was) and are no longer offered.
This article has a good introduction into the things to be considered:
http://www.candm.com.au/atm/
There are several threads on this site and others dealing with similar experiences. For example:
https://www.expandasdownunder.com/threads/had-a-mod-plate-done-today.8200/
https://www.expandasdownunder.com/threads/upgrading-atm-on-our-17-55-5sh-ob.9617/
http://caravanersforum.com/viewtopic.php?t=70598
Look forward to hearing your progress, when I get it done I'll post my experiences.
Adrian.