New pellet stove venting question

My 20 year old quad Mt Vernon AE is being replaced. For 20 years it was vented into a chimney behind my hearth. The Quad has the standard 3 inch vent pipe that was adapted up to 8” as it exited the thimble in the wall in the hearth. The chimney is block with an 8 inch clay liner. To my knowledge this has worked well for 20 years.
When pricing a new Harman absolute 63 to replaced the Quad, the dealer wants to run the 3 inch vent into a 4 inch double wall pipe thru the wall and then 4 inch up the entire chimney. The cost almost doubles the price of the stove. Is this really necessary especially since the Quad seemed to work fine for 20 years?