How to preserve MX records when switching domain registrars?

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http://serverfault.com – This may be a stupid question, but I am not an expert with DNS yet. I have created a new website for a client that I would like to host with Heroku. The client has registered their domain with Network Solutions, but the NS records are pointing to BizLand hosting. They currently use BizLand for hosting their website and email. I would like to preserve their email while switching to the new host. I need to add a CNAME record to a www subdomain in order for their website to switch over to the new host. The problem I'm having is when I log into Network Solutions (the registrar), it does not allow (HowTos)