deleting a uid from a public key

[Denis Walker]

Hi guys

According to your manual you can delete a uid from your local public key. But 
if someone else imports your key it merges the uids from the old and new keys. 
So the deletion does not take effect. The manual says in order to delete a uid 
from someone's public key you must first remove the key and them import the 
new key. Why does import not delete uids? Are there any security implications 
involved here? If I am updating keys should I always remove the key first and 
them import the new one?

cheers
denis