Euro 6 engines only allowed in Greater Manchester from next May

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RE: Euro 6 engines only allowed in Greater Manchester from next May
Can I ask please as this euro 6 term is a bit confusing to me, if a bus engine is say currently euro 5 compliant, what modifications does the engine have done to make it euro 6 compliant or is the euro 5 engine swapped for a euro 6 engine and the euro 5 compliant engine modified in preparation for the next engine swap so there is always a spare available if that makes any kind of sense.