Given that you ask about Outlook (not Express) can we assume the app is resident on a PC? If so:

POP3 email pulls in the e-mail and deletes on the server maybe. Depending on your delete preferences that may be immediately, in a specified time, or never.

IMAP the e-mail is on the server and you read it from there. If changing servers be sure to get text copies of any existing e-mails that you value. If you allow Outlook to set up a new account automatically it can find servers on (say) .de rather than (say) .co.uk but if you are in the US it will probably default to your systems and get it right. I would not trust automatic set-up but manual is not a walk in the park unless you do these things often. PORT numbers are usually the problem, ask the provider for all parameters - up front.
OR -
One way is to open the second account and forward to it any valuable e-mails. Then jump ship. Telling all your contacts first. Better still, run the two side by side as long as you can. see below.

I run Outlook 2007 (2003 was similar) with 10 e-mail accounts. One for each source of potential spam. I set it up to load on demand not automatically, some are POP3 some IMAP. E-mails get sent automatically - I can't prevent that except by "saving" and not "sending". 2003 I could control the send more to my preference.

Best of luck