Try this, I think it will work . . . If the emails are in the inbox, move them out, into a temporary folder or wherever, just out of the inbox. If the emails are not in the inbox, fine. Now, move the emails back to the inbox. BIS should see them again and grab them. This assumes you have your yahoo email in BIS.
I've done something like this before and the emails were pushed to my blackberry when moved into the inbox. Test it for yourself on a small scale before you proceed. Hope it works for you.