Better done in autumn, but you can do it now - provided you're prepared to keep it very well watered during any vaguely dry spells right up till winter. The reason for this is because it will take some time to make new roots, so for some weeks/months, it will just be a potshaped rootball sitting in the ground, and will need a supply of water periodically if the weather is dry.

As above but you need to check whether you soil in acid to neutral first.