Dynamically update existing domain

As best as i can tell, if i have an existing domain that i'd like to dynamically update (lets say i'm looking to update both http://ftp.mydomain.com and http://www.mydomain.com) I need to purchase a CustomDNS.

I have 3 questions from here?

1) What if that domain's IP registration already is 'controlled' (for lack of the correct word) by godaddy? Will part of the CustomDNS sign up process allow Dyn to take control of this existing domain from godaddy? If not, how exactly is that done?

2) Assuming that the answer to #1 is 'yes', how long does the changeover take place? and does it matter what time of day i make the change?

3) Assuming again that the answer to #1 is 'yes, Dyn will take control of the godaddy IP', then do i need to go back to godaddy and cancel the domain with them (because they are still registering my domain to a static IP that will soon be obsolete).

4) I assume that the $30/yr for a CustomDNS will include the functionality to dynamically update my DNS registrations as my IP changes. Is that a correct assumption?

Thank you for any help you can provide!