Question about retro-fitting a "built-in" wood burning fireplace/stove

Hello all, new to the forum but long time wood-burner here in the mountains just outside of Denver Colorado.

We recently decided to look in to replacing our stick-burner in favor of a pellet stove. We had an installer come out to take a look at what we had since we were concerned that we'd have to demolish our rock facing to get the appliance out (the faceplate is completely built-in to the rock facing). The installer said he could basically gut the existing unit and re-use the shell - he gave a quote for the install-only of just under a thousand plus the permitting fees for a County Inspector to sign off on the install. He doesn't sell the stoves, but suggested a Greenfire GFI55 would fit. We'd be on our own to find a stove.

The more research I did, I decided I'd rather go with a Quadrafire or Harman. A retailer who was quoting the stove didn't sell Harman so I found a dealer who did. That dealer happen to be associated with the chimney cleaning company we have used for years and so they had a file on our fireplace. After the sales person looked up the file, he informed me that because of the type of device I had ("EPA certified" or something like that), it was not a candidate for converting to an insert of any kind, pellet or wood. The entire unit would need to be removed requiring a remodel of the rock facing and internal structure. His thoughts were to stick with what we had unless we wanted to spend some $10k or so all told. He was also concerned about the installer we had come out as he should've known this. The Harman dealer does their own installs.

We're now standing down on the whole project as that figure is twice our budget.

My question is: Is he right? Is this type of appliance not convertible?

Thanks in advance!

I'm not sure what my current wood-burning fireplace is classified as. It is an RSF Energy Onyx from back in 1994 or so. It has a closed circuit air-supply and chimney. I've been told it is not an insert. Here are some pics:

Thank you for all the great replies!

To answer some of the follow up questions:

The fireplace does indeed put out quite a bit of heat. Running it on full bore coupled with a ceiling fan and our open floorplan we can keep our propane-fired boiler use down to basically only heating our domestic water and I can walk around the house in my underwear (sorry for the visual) when it is 20 below outside. Keeping the heat even is a bit more challenging however. We use our windows as the temperature regulators since even damped all the way down the fireplace can run hot. On the other side of the equation, by morning the fire is completely out, the fireplace is barely warm and the thermostically controlled blower is off. At our old house, our freestanding Lopi woodstove could keep a load of wood burning low all night and we'd wake to a toasty home.

The idea of the pellet stove was to use a thermostat control to keep the place just right all the time. We thought that with a sufficiently sized hopper and the right stove we could wake up to a toasty home again. The relief from wood gathering and the debris hauling wood in tends to leave in the living room was not completely ignored either ...

The chimney is not stone, that is just a facing - real stone from the property, but just appliqued to a subframe of some kind as an enclosure for the chimney pipe going to the roof.
I also had considered the concept of a freestanding stove essentially piped into the "chimney" and up to the roof (existing fireplace removed), but it would intrude on a major traffic path into the living room area.

The installer we had out to the house just emailed over the dimensions we had to be concerned with: "The opening dimensions for the unit to fit would be 28.5” tall by 29 58/8” wide and the surround or back plate has to be able to fit in the dimensions of 37” wide by 31” tall." So cutting the faceplate would yield even more room than that I would suspect as I don't think his plan involved cutting the faceplate - but then again maybe it did. He is the one that suggested the Greenfire GFI55. His latest response was that he would happy to research other stove options for me to choose from that would fit the dimensions if the GFI55 was not my cup of tea. If he is wrong about his approach, I would think the county inspector would catch it - of course we'd be at the point of a demo'd fireplace and no pellet insert either ... basically scrod but maybe saving the house from burning down.

I don't know what the heat output of our fireplace rates at, but I kind of had the feeling that a pellet stove would put out less. That perception is what had me looking for something bigger and led me to the Harman idea ... and now I'm feeling like we're kind of stuck with what we have since we aren't interested in a major remodel right now. But rather than gathering wood for the winter, we're ordering delivery this time. Now if I can just keep the debris down and get the fireplace adjusted so we can burn low-and-slow to wake to a toasty home in the mornings ...