Often I get the same question about when to upsize the curry leaf plant. I am not a serious bonsai mania nor professionally educated vegetable glower, however my idea is that there is no specific timing to upsize the plant, as long as it is comfortably warm season. If the plant is kept in the green house, you should be able to do so year round.
This plant has overwintered 3 times.
Partially fill the pot with clayish soil, locally known as Akadama Tsuchi. It retains moisture while large clearance between the soil allow good drainage. The picture of the soil in the pot is wet because of rain getting into the bag of soil. Akadama Tsuchi when dry is show as the picture.
Gently take the plant from the pot.
Fill the gap with the Akadama Tsuchi or planting soil. There is no specific rule for the balance or soil to fill the gap. Regular planting soil should be just fine.
After filling the gap, lightly knock the pot to settle the soil. Add some more soil if the root is exposed. Water well, then done.