The full process for moving domains between accounts is listed here:
(note the restrictions section regarding promo codes an referrers)
Since you are moving this between two existing accounts, you will want to use the instructions in the "# 4 Moving a Domain to an existing account (Old Account to Old Account move)" section.
Note that you can move the hosting without moving the domain registration, but if you do want to move the registration you handle it like any other domain registration transfer (on the old account, set the domain to unlocked, make sure whois contacts are up to date, obtain authorization code, then go to the new account and initiate the transfer).
When you move the hosting from one account to another new DNS is set up (as the IP address/Server changes), so the process detailed in the above link is what you use to move hosting or a domain between accounts.