6" liner into rectangular liner??

So, the dealer came out today to measure up my fireplace and do all the pre-sale investigation that they do. Everything went smooth except for one thing...

About a year ago we bought our house and during the chimney inspection then the sweep/nspector indicated the clay tiles were in rough shape and indicated a re-lining was necessary. This was done after buying the house, and unfortunately at the time I didn't really know what to ask and there was just so much else going on that I let him put in what he did and didn't ask enough questions or do enough research (I also didn't realize I wanted an insert. Didn't grow up with a fireplace and hadn't really experience just how little heat an external masonry fireplace put out... wish I had come here before all of that!)

So, anyway, he put in a 5.75 x 8.75 inch rectangular liner. Non-insulated I think. It was mortared in at the base and at the top. It might be insulated, as he indicated a call for an insulated liner on the original inspection, but the receipt of the work done just indicates a liner and does not mention insulation.

Anyhow, the dealer was hoping it was insulated (he's going to call the sweep to see) and if so, was suggesting that it be possible to install the 6" liner and connect it to the rectangular liner.

Is that feasible? The cross-sectional area of the rectangular liner is almost double that of 6" circular, so surely that would cause issues, no?

Is this code? Would it void warranties or anything? I'm already thinking draft might be an issue as it's only a 13' exterior masonry chimney.

The old clay flue tiles were cracked so we originally just relined the chimney to function as an open fireplace.

So, the rectangular liner in there runs from just above the original damper (which has been removed and top damper installed) to the top of the chimney. It sounds like he wants to run a 6" liner just from the top of the insert to the base of the other liner, so like a 12 inch run or so, and then connect it there. Effectively, this would mean that 90% of my liner run is the 5.75 x 8.75 rectangular liner that is already in there.

You're right Bholler, the original liner is mortared in with Chambertech 2000 (I think that's the right number?)

Anyhow, assuming the rectangular liner is insulated and we connected a short 6" piece to it, what kind of drawbacks/issues could I expect from that. Per my understanding it would result in decreased draft, right? Anything else?

Would there be any issues with running an insulated oval liner (equivalent to 6" round) through my current liner. A liner in a liner? I'd rather do this than tear the old liner out as I don't think I could even fit a 6"round in there with insulation unless I also took out the clay tiles.