I have moved my sites a few times and never really had any problems that effect rankings. If you leave both "open" until the net finds your new DNS, you will be fine.

For other, do not get so pi$$ed off with your hosting company that you "fire" them until you have a new host and that DNS has propagated the internet.

My two cents.

Michael

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Non Piercing Nipple Jewelry - All the pleasure and none of the pain! - Body Jewelry