If this is just going from a pot into a larger pot, then it doesn't really matter, as it won't be disturbed. It might be due to flower shortly (?) if it has buds on (obvious large buds at the tips of the stems) - maybe leave until after flowering, though doesn't matter. You should use an ericaceous (for acid-loving plants) compost.
A tip of mine when going into a pot not much larger, or when the plant gets in the way of you trying to get fresh compost into the small gap round the old rootball: using the old pot (without the plant) or one the same size, fill the new pot around the old one, i.e. 'casting' a hole of the correct shape/size in the new compost. Carefully remove old pot - hopefully the compost doesn't collapse into the hole (if it does, then try again). Lower pot-shaped rootball into hole - give a few taps on the ground to settle the soil, and you're done!
I have two Camellia in pots - they'd much rather be in the ground, but I haven't a suitable place for them. The pots are keeping them smaller than they should be, and they forever need watering (ideally rainwater) in summer to avoid drying out, which can cause bud-drop. Every few years, not wanting to use a yet larger pot, I have to chop off some of the old soil and roots to make room to add some fresh soil. I'm not sure all this is worth the bother...
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