You cannot ever change the wallet seed and keep the addresses associated with the wallet. The addresses are generated from your private key, so they are always linked to eachother.
The only way and most easy way, is to simply create a new wallet with a new seed in the client. Next you simply send the coins over from your old wallet to your new one and you are good to go.
Don't throw out the old wallet though, there is always a slight chance someone has you old addresses in their contacts and may accidentally send to one of those addresses. Just keep the seed to always have the possibility to access it.