Not sure I understand the Q.
IMAP will keep your msgs on the mail server and keep your mail client, WLM, in sync with it. You will keep any msgs in the mail client that you previously POP3'ed and did not leave on the server.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC