You can definitely keep your pre-existing email address when you switch to using OPP for your web hosting and email.

One thing to note, is that while the email address can be the same, and there won't be any interruption in your receipt of emails, the way that you check your email will change to using a new webmail interface. We use MailTrust for our email, and they have a fully-featured webmail interface that we think you'll really like.

Additionally, your inbox will start over from scratch --- since you'll be changing mailservers. Just make sure to get your contact list and any info that you need from the mailbox before making the change and you should be all set!