I think that what odolvlobo says isn't necessary. Essentially, he told you to include your old address in the message that is signed from your new address.
No, I am saying to do this:
-----BEGIN BITCOIN SIGNED MESSAGE-----
I am <user name>. Today is <today's date>. My new signing address is <NEW signing address>.
-----BEGIN SIGNATURE-----
<OLD signing address>
<signature using OLD address>
-----END BITCOIN SIGNED MESSAGE-----
This proves without a doubt that the original <user-name> want to use the new signing address.
As I wrote above, signing messages with today's date using both addresses is probably sufficient assuming the post is quoted, but it is not a cryptographic proof, and it is actually more work than what I propose.
Actually, if you feel that simply quoting a post is sufficient protection against changes, then this is sufficient:
-----BEGIN BITCOIN SIGNED MESSAGE-----
I am <user name>. Today is <today's date>.
-----BEGIN SIGNATURE-----
<OLD signing address>
<signature using OLD address>
-----END BITCOIN SIGNED MESSAGE-----
My new address is <NEW signing address>
But, note that simply moving the new address inside the signed message provides so much more protection.