Well you can do that - but it wont' benefit the tree, it can't grow any bigger in a pot that size - its not the compost so much as the fact it can't make roots sufficient to support more topgrowth, or even, in fact, the topgrowth it currently has. so that's a no, really.

Unless you intend to 'bonzai' the tree. In which case you can take the tree out (late spring) cut the roots back by 30-50% and repot into new compost mixed with loam. Prune the top as well to keep it in balance and ensure you feed and water well. High nitrogen feed. This will create a smaller plant that will have smaller leaves etc. But ultimately a tree will one day need to go in the ground or in a much bigger pot. Why not grow plants that will only get to the size you have room for? Smaller shrubs like Hebe, Euonymous, Box and dwarf conifers.

thank you :) Will get a bigger Pot !!
Bamboo: thank you, I want the tree to benefit so a bigger Pot it will be.
Hoya105: This tree thingy is a memory for my OH and a challenge for me!!!

What Hoya said, but - 50% root removal is far too much. Bonsai means tree grown in a pot. With yearly pruning it will happily live in the same pot for ALL its life (Sorry Bamboo). ensure compost is very free draining or use non clumping clay cat litter.