The 'split' depends on what the particular country you are flying though classes as 'upper' and 'lower' airspace. In the UK it is now FL195 as it is in most of Europe but previously it was FL245. An upper air route may be slightly different from its lower airway in spite of the numerical value being the same as the airway in lower airspace may need to deviate slightly to avoid (say) a danger area which might not extend into the upper airspace..