Water heaters are heavy, and moving heavy objects without experience can lead to injuries. As of 2015, a majority of water heaters use gas power; an improper replacement can lead to gas poisoning or fires.
Even when drained, water heaters are heavy. In addition, their large size makes them difficult to move. Proper care must be taken when removing a water heater to keep it from falling over. Back and knee injuries can occur if improper lifting technique is used when loading them. Similarly, caution must be exercised when moving the new system into place.
Older pipes can become rusty and worn over time, leading to potential breaks when replacing a system. Even tiny scratches can lead to tetanus, so wearing thick gloves while working with old pipes is recommended. It's also wise to let the system cool down; hot water splashes can cause severe burns.
Perhaps the biggest risk, however, comes from dealing with gas connections. While large flows of gas make a sound, small leakage can fill a space quickly, leading to gas poisoning. If the space has a source of ignition, the room can flashover if gas builds up. While ventilation can mitigate this risk, knowing how to ensure a gas line is completely disconnected and constantly looking for signs of gas buildup provide better protection.