How can I make exchange check a pop3 account when a user is on web access?

We are in the process of switching from a web based e-mail service to our own exchange hosted e-mail.  Instead of doing a direct import and changing the DNS entries we are staggering the transition, doing groups of 20 users at a time.  This means I cant change the DNS entries all at once so our email is still getting delivered to our old web host.  I have configured some users Outlook programs to check the old POP3 account and save their new mail into exchange.  This works to a certain extent, a user can open outlook, download their mail, and then view it through web access on any computer.  The problem here is that they will only recieve new mail (from the old POP3 server) when they open the outlook instance that I have configured to check their old account, (aka, no new email if you only want to check through web access).  The decision to do this transition incrementally has already been made so I would now like to figure out a way my users can get new e-mail that is being delivered to a remote e-mail server without opening outlook.  This is also going to be a pain when the transition is over because I think that the pop3 accounts in outlook will start throwing errors once we don't have service with them anymore and I will have to go to every instance of outlook and remove the pop3 account.  Any help appreciated. -Woodie