There's a lot of good information about citrus composts ont'web but I will add this rarer advice:
Citrus roots don't have root hairs, so they can be incredibly tricky -
a) when re-potting the soil tends to fall off the roots (especially on young plants), and they won't appreciate a sudden change of soil.
b) even more importantly, they have to rely much more on mycorrhyzal fungi - so don't remove all the old soil and replace it - even diluting it is risky.
c) they are very fussy about moisture* - and recent research has emphasised that two types of soil next to each other can upset drainage across the boundary - so I'd recommend making the new soil as similar as possible to the soil it's happy in and creating a layer of gradual change from the root ball to the new stuff.
That's assuming it is happy - I've bought citrus that were in a claggy sandy clay - presumably from a nursery in a hotter country, and others that had been potted up a size using sodden soil-less compost - a quick and deadly trick - presumably to get a better price for old stock.
*The simple solution to moisture is a capillary mat system - it makes it easier for the plant to find moisture when it wants it, and also easier to drain excess moisture. This may sound odd, but a capillary system mirrors the soil system - all plants in well watered and drained soil have access to water via a capillary link to the wetter levels - while this link exists it speeds up water movement in BOTH directions.
It doesn't have to be expensive - an old slate on a bit of expanded plastic in a tray is the 'shelf', and the mat covers it and hangs into the water in the tray (hanging over one side is OK, 2 is best)
Before I used capillary I lost about 20-30% of my citrus every winter, now I lose a few percent.
Having said that, Kaffir Lime is one of the tenderest and fussiest citrus on sale in our climate.
Good Luck
Cheers.
