Changing liner size?

Hi all, hope your summer was good. I need some input on maybe replacing my clay liners in the chimney(not this year but maybe in the spring),I replaced the 12" liners and added 2' to the height when I switch to coal due to the fact most were cracked from a fire(previous owner and his wood stove),the chimney has a great draft in colder weather but on the warmer days not so good so my plan was to remove the 12"SQ and replace with 8" round and fill the air space with vermiculite hoping this would be better but I'm not sure what to do at the start of the first liner(at the top of the smoke chamber)the old one I broke out was not sitting on a shelf just the first 10" of liner was secured with mortar(between the brick and the liner),I mortared the whole 24" liner when I replaced it now if I were to put in the 8" liners do I remove the old 12" liner and install the 8" the same way, mortar the round into the square or get a 1" or 2" thick pc of stainless(with a 8.5" bore and blocks Tig welded to keep the round center over the bore) to sit on the top of the square liner and build from there? now this is a insert in a brick fireplace and I do use a pipe from the stove up in to the first liner with baro in it, I will change the pipe to a double wall smooth bore(the pipe I got is a P.O.S and will never buy that kind again) but I am just looking for some feedback right now,Thanks and have a good one. Keepaeyeonit

Is this an exterior chimney or interior?

Different solutions for each, Me thinks.

Sorry,It's on a East wall and It's a exterior chimney.Keepaeyeonit