If you use Outlook, then all emails are on your PC, and not on the ISP system.
As soon as you downloaded them to view them they are removed from the ISP.
Therefore I don't really understand your question?
If you change the settings in Outlook to the new ISP, nothing else needs to change. The emails will come from a new ISP, but the inbox etc in Outlook will continue as if nothing has happened. Your contacts will not know there has been a change, so all addresses should function as before.
Think of it as changing your street address number. The contents of your list of addresses inside in the drawer is unaffected.
The only way you can protect yourself from the new email address getting picked up by the many viruses and worms is never to use the new address. Rather defeats the purpose.
Once you have sent an email to anyone else, if their PC gets infected, then you can get targetted by the virus, which may look as though its been sent by you, even though it was someone elses PC that was infected. Thats how these viruses work, they "pretend" to be from an address picked at random from an address book on an infected PC.
Some simple efforts can help though. NEVER forward emails with a long list of recipients, NEVER permit family and friends to do the same when your address is in the list. These lists are too easily harvested by a virus on any of the recipients PCs.
You may take great care to keep your PC clean, but you cannot control others that may have your address in their system. Thats the danger.