Oil to Gas Conversion
I think you are saying that you have a 50 gallon indirect hot water tank that uses hot water from your oil fired boiler to heat the water. Is that right? If that is so, it should already have its own zone off the boiler. If all you are doing is swapping your oil fired box for a gas one, I don't see why you would need to replace the water heater.
If on the other hand, your water heater is oil fired, it would have to be replaced with a gas fired water heater.
I am thinking none of the pros have answered because your question is a little vague.