Which European country entered World War I as part of the Central Powers but later switched sides to the Allies in 1915?
I think you mean Italy.
Italy was part of the so called "Central Powers" but didn't enter the war (beside Germany and Austria-Hungary) remaining neutral (the clause of the treaty called for support against an aggressor not to be the aggressor) up to 1915.
In Italy there was a deep division between the Church and the monarchy on one side sympathizing for the catholic monarchy of the Hapsburg and the idea of the Risorgimento (the movement to get back the territories still under foreign occupation) on the other.
At the end the latter tendency prevailed (helped by promises of land acquisitions by the allies).