The creation of the Australian II Corp in northern Malaya.
The historical move to Johore was most likely made to ease administration but also heavily influenced by the need to give the Australian commander Gen. Gordon Bennet his own private fiefdom with no need to deal with British and Indian Army troops and commanders. This was important as Bennet was more than just a divisional commander but also the Australian Imperial Force commander in Malaya and therefore had direct line of communication to the Australian Government. He was notorious for his own self interest and self advancement.
In APOD to have build up of Australian forces in Kedah and the creation of an Australian HQ above him will require a political backstory as Bennet was one of Australia's most senior officers with both powerful friends and enemies inside the Australian military and Australian politics.
The deployment of the Australian 8th Australian Division to Kedah is not unrealistic as while the division (division comprised of Div. HQ, 22nd Inf. Bde and very little else) was in Malaya Command reserve in western Malaya a careful examination of Penang Island was made to allow for training areas, barracks, hospitals, workshops, etc. It was found that Penang Island was suitable for basing the division even after additional units arrived from Australia.
The sending of a Div HQ and the 1st Arm Bde of the 1st Australian Armoured Division fits perfectly in response to the Japanese movement into Thailand in April 1941 in APOD. To fill out the understrength Australian infantry division and armoured division can only be done by the addition of UK units and the creation of supplementary units from the bloated Australian reinforcement depots and over strength Australian units. Thus an Australian commander who can work very well UK and Indian troops and commander will be essential.
separate to above
As the 17th Indian Division and its 46th Brigade and all the other units historic units that served in Burma Have been included in APOD Burma operations, the British 18th Division would have to go to Singapore/Malaya and not Burma as in APOD.
The sending of the 18th British Division to Malaya/Singapore rather than additional semi trained Indian formations would be a very powerful political move to reassure Australia. The 18th Division was victorious in full scale battle exercises in the UK to gain the right for overseas service. The division was reputed to have excellent administration. Even though the division was a "second line" territorial division most of its infantry battalions were either first line (pre-1939 expansion) units or their "daughter" battalions of the March 1939 conscription duplication. Thus each battalion had a very large proportion of pre-war volunteer weekend soldiers from close knit communities and the majority of the conscripts were from the same general areas. Also the quality of the men was good as most of the troops came from rural counties (3 battalions from Norfolk; 2 Battalions from Suffolk; 2 Battalions from Cambridgeshire) including many from in and around the university and town of Cambridge. Historically the 18th Division in WWI had an good reputation starting with the first day of the battle of the Somme when it took its initial objectives (very rare feat of arms on that terrible day). In both world wars the division had popular, intelligent commanding officers who strived their best for their men.