Gauge conversion of an R class was not a simple as many seem to believe. There was a heap of design work necessary as 'designed to be gauge convertible' was something of an overstatement.
I have doubts that 766 will ever be accredited to do anything anywhere especially being located in NSW.
In every book I've ever read about the R and J classes they always make special mention that the two locomotives were designed to be gauge convertible without mentioning how this was supposed to be done.
What is involved in gauge converting a locomotive?
Why are some locomotives gauge convertible and others aren't?
What other locomotive types could be converted?
In the case of the R class, it really was easily convertible, as shown by R 766.
The R class was built with the bar frames spaced at a separation suitable for standard gauge. The cylinders were cast to be suitable for standard gauge, but were bolted together (and to the frame) with spacers so that they sat at broad gauge spacing. The axles were all of broad gauge length but the bearings on the axles sat at the standard gauge spacing dictated by the frame spacing.
So basically they just removed the spacers and moved the cylinders to SG spacing and after removing all the wheels, either shortened the existing axles or had new standard gauge axles made and reassembled the locomotive. In theory hangers for brake gear were provided for both gauges. The brake gear would require modification to get the same forces at the new gauge.
But basically, not too hard.
The post war J and N class were also designed for conversion, but these had plate frames which were spaced at the usual broad gauge spacing. In this case the actual cylinders were the same (or very similar to) normal broad gauge locomotives, but the spacers between the frames (often called "stretchers") had spacers each side to hold the frame plates at BG width. So unlike the R class, an N or J had to have the frame dismantled and re-assembled as part of the conversion. In the case of the J, the supports for the air reservoirs between the frames were not fitted with spacers and would have to be modified. After the frame had been re-assembled at SG spacing, the process of modifying the wheelsets would be the same as the R class.
The SAR had a different process with "dished" wheels. With the wheels turned with the "dished" side inward, the wheels were at BG spacing. With the dished side outward, the wheels were at SG spacing. A little thought will indicate that the tyres would have to be removed and replaced facing the opposite way for this to work.
This process was carried out on the SAR 740 class where the wheels were assembled flat side out, compared to the CR L class that had the dished side out, and the difference can be seen in photos of the locomotives. This meant that the cylinders were always at BG spacing, which meant that the L class couldn't run in NSW (who might otherwise have purchased them.)