Most times, the chain wears, goes out of pitch, and ruins the sprockets. If you pay attention to the chain condition you may be able to avoid wearing out the sprockets. Also, never put worn chain on new sprockets, since that will wear them out quite quickly.
The first question is whether the sprockets are worn. If they are, you will need to replace both chain and sprockets. If the sprockets are not worn, before putting the mower into regular use you still need to remove the chains and check them for wear. If they are showing signs of wear but the sprockets are still good, replace the chains immediately before they ruin the sprockets.
To inspect a chain for wear, turn it on its side and note how much the center of the span of chain sags below the two ends. Sag is caused by wear on the pins and side plates. Wear on pins and side plates increases the chain's pitch. Increased chain pitch ruins sprockets.
