I have dealt with this twice this year. One unit "we" installed, another some other jake leg installed.

If you run LP on a furnace set up for natural gas it will work, for a little while, apparently.

Soot builds up and eventually rolls out on every ignition.

Then, you can not simply convert the furnace, you must also blow out the heat exchanger.

The way I learned, after the first time, was to disconnect the igniter, call for heat, and blow nitrogen through every tube.
p.s. turn the gas off

The draft motor will blow a good portion of the soot out of the house.

Now remove the induced draft motor, clean off the squirrel cage and manifold. While the IDM is off, also blow backwards through the heat exhanger. If you put a blanket over the furnace while doing this, you'll prevent the big stuff from going EVERYWHERE.

After this, no roll outs after the conversion. I tried to get away with not doing the H.E. cleanse during the conversion and had to come back... don't do that to yourself.