Repotting an Overgrown Shrub
All permanent shrubs need repotting into a bigger container. This gives the roots space to grow and an energizing "meal" of fresh potting mix.
From: DK Books - Containers for Patios
Remove the Plant
Lay the pot on its side, ask someone to hold it and gently ease out the plant by pulling its stem. If you’re in danger of damaging the plant, or it is stuck, slide a long kitchen knife around the insides of the pot to free the root ball.
Pry Out the Roots
The roots will probably be in a tightly congested lump, in which case use a hand fork to pry out the encircling growth and shake off the old dead soil and surface moss. Aim to create an open spread of roots.
Cut the Main Anchoring Roots
Cut back the main thick anchoring roots by up to one-third, but leave the thin, fibrous roots unpruned. Pruning promotes the growth of more thin roots, which absorb moisture and nutrients.
Place New Plant
Replace the old crocks in the base of the new, bigger pot, pour in some potting mix and position the plant. Once the plant is centered and upright, pour in more potting mix, firming it down, and water in well. Top off with gravel.