What is gas cost for update operation in Ethereum?

Recently, I have read a ICDE paper about Ethereum, it states that:

the gas cost to update a word to storage is 5,000.

But according to Ethereum: A secure decentralised generalised transactionledger, the gas cost of reset operation is 5,000.

Paid for an SSTORE operation when the storage value’s zeroness remains unchanged or is set to zero.

Based on common sense, update is at least as hard as store itself. So, I think the statement of that ICDE paper is wrong. Can anyone help to figure this out?