My amana gas furnace was not converted to propane when hvac guys instlled it and has been running on propane. I've used 600 gallons of fuel and have repeatedly complained about strong odors, black sutte exhaust vent, and high fuel costs. When the problem was discovered I was told just to do the conversion and everything would be OK.
I question this.

The system will need a good cleaning. You will have soot build up inside the heat exchanger, collector box,inducer wheel and housing. If not properly cleaned after conversion the restriction will get worse.

If the furnace isn't sooted up too bad, it may be okay. I have seen units not converted soot up so bad that it clogs the heat exhanger, draft inducer etc, and at that point your best bet is to replace the complete unit. If the unit has been looked at by a pro, and it is operating correctly, you are probably okay (got lucky) with only 600 gallons through it. Just keep your eyes open for things like plugged drain system, or tripped flame rollout switches. If either of these things happens within the next few months, I would definatley raise my concerns with the installer. Other than that, your probably fine.