I remember actually finding it very easy (and fun!) to hack my stupendously detailed X-wing LEGO set when I was little; the main difference was that the mix-and-match would generally take place with larger units (partial assemblies from smaller "model-set" type sets) or involve adding little bits coming off of things (flames from some fantasy castle set went onto the barrels of some generic set and made extra rocket engines to go onto my X-wing!)

On a side note, I remember a LOT more of my childhood play than I thought.