Hi Mita, it is possible to do this, and there shouldn't be any major issues with doing it. Here are a couple of threads discussing it, and there are others if you search for "change cname".
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PkcRAAS
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PdpcAAC