Do I need to replace existing 8" liner with 6" to install a fireplace insert?

I recently bought a 103 year old cottage with a big fireplace that was boarded up. It turns out the previous owner had an 8" metal liner installed in the chimney. I think my best option would be to go with a fireplace insert, but I was told by the guy that the chimney company sent out to look at it that they would have to remove the 8" liner and install a 6" one. I would have thought an adapter could be used to step up from 6" at the insert to the 8" liner. That said, I am ignorant about fireplaces and chimneys, so I am willing to accept there may be a very good reason.