At some point in the future you may want to chop below the first branch anyway to encourage a burst of new branching, so it might as well be whilst you dig it. It is not common for a Casuarina to die back to the base if you trunk chop below the branches. However, it is a very small possibility, depending more on the species of Casy. Between October and Feb is a great time to do what you plan, so go for it.
And as already stated make sure you keep the water up, especially with these sweltering conditions we've been having around Syd.
Try to keep a lot of fine feeder roots on the tree, but with Casuarina in the ground they notoriously don't have many close to the base unfortunately. Do your best, and it will be fine to go for it regardless.
One word of advice: If you plan on trunk chopping before you dig it out, leave enough room for you to grab hold of it whilst you yank it out of the ground. Don't grab it by the base, as depending on the species you might damage the beautiful aged bark. Some barks are as hardy as Chuck Norris, whilst others will crumble from grabbing it.
Good luck, and put up pictures.

They make the best bonsai in Australia
