New drolet ht2000

Hi everyone, I have a couple questions for anyone more experienced than me. I pulled out the old open wood stove I've been using and am installing a ht2000. My old stove and chimney are 10" pipe. The new one has 6" pipe. I know it may cause draft issues and maybe the chimney will get dirtier faster. Is it worth a try to go from 6" pipe to 10" chimney, I don't really want to spend the time replacing the chimney unless I have to. If I need to replace it, can I(should I) leave the 10" in place and just slide a 6" down the middle?
Any tips or advise will be appreciated.
Thanks

It is air cooled triple wall, going through the wall at least, the chimney is the same od, about 15 1/4", but haven't taken it apart to check.

Would it be ok for me to simply use a flexible chimney liner through the entire length of my chimney?