Make WordPress "readonly" on a given site

Question

I'm moving a WordPress installation from one server to another. The IP address will change, but the URL will not, since I will redirect DNS.

The problem: while DNS changes propogate, people might end up at either installation.

That's fine for reading, but if they leave comments/register/etc on the old server, their comments/registration/etc will be lost.

Is there any way I can make WordPress "read only" on the old site?

I realize WordPress uses its database (MySQL in my case) for internal purposes, and I can't simply disallow database writes.

In other words, I want to disallow user-initiated database writes/changes, but nothing that will stop WordPress for working properly in read-only mode.

Ideally, each page on the old server would say something like:

"You are viewing this WordPress blog on an old server. Until you receive a DNS update with a new IP address for wordpress.foo.com, you can read articles, but cannot comment on articles, register as a new user, etc".

Answer 1

Turn off commenting in your old blog using the following Plugin http://wordpress.org/extend/plugins/disable-comments/

Then turn off the new user registration from Settings->General by unchecking "Anyone can register"

Hope it will help :)