Changing to a new domain

Hi,
I've searched alot but couldn't find what I want, so I am posting here hoping someone will help.

I have a site running on wpmu and currently I have almost 800 blogs. I decided to change my current domain to a new domain e.g currently all my blogs are on blog.com, my new domain is blog123.com.

What I want is to transfer all blogs to the new domain (blog123.com) and allow blog posts on the old domains to resolve to the new domain.

Old domain post: http://englishmusic.myblog.ir/post/96
New Domain post: http://englishmusic.mihanblog.com/post/96

So if you click on the above link "Old domain post" you will be forwarded to the same post on the "New Domain post", that's exactly what I want.

appreciate any response. thanks in advance.