Just a question; if you use up all the oil in the underground tank, wouldn't it then be OK to leave it there so you don't have to dig it up?

I have the same situation. So here is my 2 cents.
Check with your town/village buildings department. Depending on how old the tank is and the building codes they may have records of when the tank was installed. This will enable you to find out the tanks construction and age. Whatever you do, do not pressure test the tank! If there is a leak or weak point in the tank, pressure testing will only make the problem worse.
Until you make a decision to change (the tank or convert to LP), check with your oil supplier the may provide moniroring service. They will rate the efficiency of your burner and monitor your oil usage. This will allow them to tell if you are losing oil.
Aside from oil being cheaper, I find it to be more comfortable.

yes, propane is more money. oil has much more btu content than propane. and is usually slightly cheaper

i bought a house, 1st year heat/hot water, 3400 bucks. converted to oil, 2nd year 764 bucks. installed a buderas boiler w/ecomatic control, and hot water mate.

hope this helps.