How to accurately judge economics of converting from oil heat to liquid propane?
First, check to see if there are any tax breaks on the LP system for your area.  

My rule of thumb on energy savings savings is, it must pay off in less than 10 years or half of it's usable life (which ever is shorter).  Since you have to replace the unit, see if the difference in cost - expected savings in energy over 10 years justifies going LP.  A lot of times, the energy efficient items don't pay off unless they have a tax break or you have to replace an entire system anyways.

Edit:  I was curious about the results and looked up the values.
Assuming you are using 800 gallons of oil each year at a rate of $4.21 a gallon = $3368 year.
You would need 1060 gallons of propane to match the 800 gallons (taking into account different efficiency) at a rate of $3.80 a gallon = $4028 a year.

Cost data came from here: http://www.eia.gov/dnav/pet/pet_pri_wfr_a_EPLLPA_PRS_dpgal_w.htm
BTUs of propane: 91,500 per gallon
BTUs of heating oil:  135,000 per gallon
Conversion from oil to propane used:  oil gallons * oil BTUs * oil efficiency = 92,880,000 effective BTUs used each yet.  92,880,000 / propane efficiency / propane BTUs = 1057.38 gallons of propane used.