' EAS is a measure of the dynamic pressure exerted on the aircraft. This dynamic pressure plays a key role in the lift and drag created by the aircraft. For a given EAS the aircraft feels the same dynamic pressure, and therefore lift and drag, regardless of altitude. The higher the density altitude, the thinner the air, and the faster an aircraft must travel through the air mass to obtain the same EAS. The actual speed of the aircraft through the air mass is called the true air speed (TAS).'
So,if at FL180 your Vmo will be ,for ex,300kts, if you climb,and want to maintain this IAS the result will be in an increase in TAS,resulting in disruption of airflow over the wing.This is why you'll have to decrease the IAS,in order to mainatain a maximum TAS..your ex of 248kts at FL 250 will corespond to the same TAS as Vmo at FL180....I hope I've not mixed it up,I'm a bit tired now..
Brgds..
Alex