Just to make sure: This is not a recent puicture, but of a year old, right?
If it is recent; You are too late to repot. And it is sitting in the wrong spot (It should still be dormant).
As for repotting.. You have to be carefull removing the old soil. If this has been in this composition for 2 years, it has not grown enough for the roots to merge properly. You can lift it from the pot, and carefully remove some of the soil.
Or.. If you dare.. You could do as bob suggests.. Take the whole thing apart and re-create the forest. Currently it is a little .. contived. Have a look online for bonsai grove and study the best groups you come across. A variation of sizes, clumping trees in little sub-clumps will make this a much more atttractive composition.
e.g.
www.pittsburghbonsai.org/wp-content/uplo...-collection-Bru1.jpg