If, after six months, the storage user decided to flag their data as immutable, then they would need to add the difference between what is currently staked in the smart contract and what it would cost to immutably store their data. So, a user with 1gb of data stored and .5 $SHDW staked in the smart contract (after six months of staking) would need to add .5 $SHDW in order to make their data immutable.