Yes, she needs companions.
How did your other Corys die? How many? What are their tank mates? What species of Cory do you have?
I would like to add that Corys can often live up to 20 years.
There are several methods for changing gravel to sand.
First of course is to wash the sand well. I find doing small amounts at a time to work best.
My method of changing gravel to sand would most likely depend on the size of the tank, the inhabitants, the decor and what would be stirred up when I removed the gravel.
The easiest, if the substrate is deep and the tank well stocked with lots of fish, might be to bucket out most of the water, put the fish in the buckets with a heater if necessary, and then remove the old and add the new. Then just add the fish and old water back. If the substrate is shallow and easily removed lower the water some, and take the old out and in with the new.
A plate or something is sometimes used as a deflector so the water or substrate being added does not plop into the tank. As far as I know I have never lost a fish from changing substrates with the fish left in the tank. Well, I don't leave Bettas in their tanks during even water changes.