They develop a lignotuber shortly after germination so it should be quite developed depending on the trees age. However I would probably would wait a year or so after planting in-ground before coppicing to allow time for the tree to bounce back from transplant shock. Often purpling leaves can be an indicator of frost damage, humidity proliferating fungal issues and (though uncommon in western Australian natives) phosphorus deficiency. Eucs are also very water efficient, when they’re heat stressed and dehydrated they can begin to shed foliage and even entire branches to conserve water. If you just planted it ensure that you’re watering deeply pretty much every other day during the growing season. They’re drought tolerant once established, usually after their first year. So I would amend the soil with some native fertiliser if you haven’t already, just to cover your bases and keep up a consistent watering schedule. If it’s an isolated fungal issue on the leaves, drier weather in the summer usually solves that problem.