Recommend a Tree to grow in a pot for patio

When you take the tree out of its pot, the compost should come with it. the rootball should be reaching the edges of the pot so it should come out in one "lump". If there are a lot of roots circling around the outside you can gently tease them out and spread them into the new compost, otherwise leave them alone.

Make sure you give it a good soak first - the best way is to dunk it, in the pot, in a bucket of water and hold it down until air bubbles stop coming out and it stops trying to float. Then lift it out and let it drain a bit before taking it out of the old pot and planting it in the new one.

I agree about the polystyrene. It's sometimes recommended to fill some space without adding weight but apart from being horrible indestructible stuff, you need weight in the pot because those "lollipop trees" are top-heavy.

Make sure that the new pot's drainage hole(s) is/are kept clear - the usual way is to stick a piece of broken plant pot ("crock") over it before putting a layer of small stones/coarse gravel, then the compost. John Innes No 3 is best for long term planting but No 2 will do if you can't get No 3 - it just has less fertiliser so you might need to start feeding sooner. If you don't have crocks anything similar will do, or even a bit of fine mesh or sturdy non-rottable cloth (I once read somewhere to use a j-cloth). The aim is to let water run out but prevent the gravel/compost from washing down into the hole(s).