Personally, I've transplanted tons of stuff in bloom and just used superthrive to keep it from getting any transplant shock. If you can't baby it though just wait because you risk losing everything if you don't do it just right.

Hope that helps :)

If it is 5 years old and in the same pot, I would not wait another minute to put it in the ground. You will find it sooo much easier to deal with inground. Just be careful not to damage the rootball; I prefer to cut or break away the pot; because if you try to pull it out, you will lose roots.
Be sure to thoroughly water it in and do not fertilize it for 2-3 weeks; the absence of food will encourage the roots to grow seeking food; and after the roots grow the leaves and branches will grow and they need food. Inground you will only need to fertilize it 3-4 times per year with a good citrus fert. with a 3-1-2 NPK ratio plus Calcium, Magnesium, Iron, and Zinc.