The Puffing Billy in Australia is regauging a 2 ft. gauge NG-16 Garratt to 30 inch gauge. The entire process has been documented with photos on their website.
https://puffingbilly.com.au/news/workshop-blog/a-giant-takes-shape/
As far as I can tell, the frame received new spacers, widening the frame. The same wheels were mounted on new, wider axles. The cylinders are not one piece like a traditional locomotive so they were just mounted as usual (as near as I can tell). Other things were widened as well.
The locomotive has gotten a complete rebuild and it's been an interesting process to follow.