New Exchange 2010 & MX record required

A new company (companyB) was started under the umbrella of an existing company (companyA).  CompanyA had an existing AD forest/domain and an existing Exchange server.  An Accepted domain was added to the exchange server to allow email to be sent/received from companyB (user@companyB.com).

Now companyA is moving office and at the same time moving to office365.  They are leaving their existing office and AD/Exchange infrastructure to be used by companyB.

I would like to know what changes need to be made in order to allow companyB to reconfigure the infrastructure to reflect their own company name.  Currently owa is pointing to https://mail.companyA.com/owa, as do the autodiscover DNS records.  Company B's MX record points to mail.companyA.com.

As I understand it the following changes will be required:
1)  Create and apply a new SAN certificate from a 3rd party CA including the mail.companyB.com name
2) Create a new A DNS record: mail.companyb.com and repoint the MX record to mail.companyB.com
3) IN EMC Server Config> Client Access > set a new external url for activesync and OWA to mail.companyB.com

Should this cover everything?