I don't think Hotmail will let you forward from one address to the other, if you really want to change the address, I would just create a new one (Gmail is miles better than Hotmail) and change all your contact details over to the new address. You can send a group email to all contacts telling them that you've changed it, and you can still use the old one for MSN.
It's still a good idea to check it every now and again to see if there's stuff there you want, but over time you'll stop using it anyway.
