It sounds like it is in a 15 gallon (60 liter?) nursery container. Assuming this is the case, you can saw off the bottom third of the root 'ball' and loosen the roots from the other surface, maybe removing a centimeter of soil. Personally, I would focus first on getting rid of the nursery soil and might then half bare root it (i.e., clean out all the nursery soil on one side of the trunk), back filling with my favorite substrate.
I would keep as much foliage as I possibly can, removing only what I absolutely must to be able to manage it (foliage powers root recovery). Being in a USDA zone 8, I think you could HBR now and, if all goes well, again next spring. Of course, you wouldn't do the second HBR next spring if you don't see strong growth beginning - you would delay to about this time next year. But, if it goes well, you might be able to begin reducing foliage enough to take the first steps in styling it. a year from now.
Not exactly an answer to your question, but what I would do with it in your stead.
There are, of course, ways other than mine or the highway.
The direct answer to your question is often given by Adair M :
it depends
Regardless, have fun with it
- sounds exciting!