Propane fireplaces use a thinner version of stove pipe than pellet stoves require in the same diameter, but it isn't tough enough to handle the soot and heat coming from a pellet stove. The vent pipes from your propane device feature an inner wall of aluminum, according to the Nevels Stoves website, which will corrode within a few weeks or months of regular use. You must install steel-lined vent pipes before putting in a pellet stove. You may be able to pull out the existing pipe and use the same opening.
Most propane-burning devices use direct ventilation, which involves a short, horizontal section of pipe running directly out of the exterior wall closest to the fireplace. While this will work as a beginning for your pellet stove ventilation system, simply running a pipe out of it won't work. Pellet stove chimneys operate best with vertical sections of pipe that rise at least 3 feet in one section before running horizontally, according to manufacturer Simpson DuraVent.
Unlike wood-burning fireplaces, gas appliances are often set into the wall or have only a decorative hearth around the opening. A pellet stove requires a safe place to sit that won't catch fire. Leg kits raise the stove up, but the heater will still require a slab of concrete or fireproof stove board below it, according to "Natural Home Heating: The Complete Guide to Renewable Energy Options," by Greg Pahl. Avoid adding a hearth and having to fill in the old fireplace by choosing a pellet-burning insert that fits inside of the opening instead of a free-standing unit.
A pellet stove also gets hotter on the outside of the device than a gas fireplace. This makes it perfect for winter heating but changes where it can be installed. Adding stove board or reflective cement board on the walls around where you plan to install the stove is necessary to prevent fires from occurring inside the wall if you want to place the pellet stove closer than 6 inches to the wall.