legacy domain question

i am trying to understand the legacy host name setup for coexist mode (ex2007/ex2013).  is that just for webmail, outlook
anywhere and phones (active sync)?

right now:
public DNS: mail.domain.com points to 'my public IP 1' and my firewall sends it to ex2007 server for port443
internal DNS mail.domain.com points to ex2007 server's IP

create:
public DNS: legacy.domain.com, point it to 'my public IP 2' and my firewall also sends that to ex2007 server for port443
internal DNS: legacy.domain.com, and points to ex2007 server's IP.

get a new UCC cert that includes the new name legacy.domain.com and apply it to ex2007 server?  and later apply that same UCC cert to ex2013 server?

(at this point, a user could go to legacy.domain or mail.domain and everything would work the same)?

install exchange 2013 and configure it.

at my firewall, change public IP 1 (the original mail.domain.com:443) to now point to ex2013 server?

users go to mail.domain.com like normal.  if my mailbox was on the old server, i would be redirected automatically?  all of my

phones, outlook anywhere clients and webmail users all still function normally?