For example, we amended the AIP so "left" had the meaning "left" as in left and right, as opposed to left meaning leaving an altitude or flight level.
But it took a real accident and a string of really serious near-hits and the resultant inquiries to do it!!
What was that accident and the "serious near-hits?" Now we say "leaving" an altitude even though we may have not yet vacated that altitude. But the intention is to leave that altitude when it suits. The old terminology was more precise in that you called "Left FL 310" for example after the climb or descent had occurred you were definitely on the way up or down. I never heard of anyone confusing left and right as in heading as against "Left" and altitude.