When the new wallet is released... What is the risk of someone using the old wallet. Will the old wallet stop working?
The risk is that the original developer may try to transfer his premined coins to another address, which will result in a fork. Those then with the new wallet are protected as their network/blockchain will not accept any transactions of those coins, but those with the old wallet may accept the other fork of the blockchain. Basically the same risk that exists with all forks for all coins. If that happens, they will need to take their wallet.dat, upgrade to the new wallet, and redownload the blockchain (it's not the end of the world, just an inconvinience).
In 2 months, when we decrease the PoS rate from 100% to 80%, the old wallets will continue at their 100% staking, so there will be a mandatory fork then, and the old wallets will become totally incapable of being on the right network.