"Upsize" liner

Hi-
Is there a problem going from 6" to 8" liner? We have currently 8" SS liner. New stove whatever we get will likely be 6", and the old 8" is good as new. Can we just use an adapter to get to the 8"?
I know, I'm a cheapo, thats one reason I burn wood! All the free firewood one wants out here just for the cutting and maybe a case of cold ones or a steak supper.
thanks!
SLK0217

Not sure what stove yet, but like th elook of the Napoleon insert better than the modified stove look like a buck, but have not bought one yet.
This is a huge old fireplace, black rock, big firebox. Chimney is about 16'. Flu length 2 days, allowed time for reflection (which was not pretty either)!
SLK0217