Question about Outlook Express

My entire Inbox was previously synced to my Mac -- all of the messages and attachments were downloaded. Because of this, Outlook has become very slow, and I would only like to keep the headers (and download only the headers of future e-mails). However, when I set "download headers only", will Outlook go through my inbox and delete all of the e-mail contents, and just keep the headers, or will I have to remove my account and set it up again with the "download headers only" to re-sync my Inbox with just the headers?