Hi there,
I have a 75 gal water heater.
The propane gas guy said that it was natural gas even though it came from a church that was using propane. And they had it set up for propane. Is that a possibility? And is it safe?

Conversions are generally easy.

This link should answer your questions:

HowStuffWorks "When buying gas appliances, why do you always have to specify whether you are using LPG or natural gas?" (http://home.howstuffworks.com/question197.htm)

Propane burns about 2x hotter than natural gas.

How would gas man know unless he opened valve to see? Or did he?