Q. Will I be able to access the server via FTP and via IP address prior to changing the nameservers ?

Ans :- Yes, you can access the server through FTP or an IP address prior to changing the nameservers. And you can preview your website with a temporary URL.

[ http://ipaddress/~your username ] before changing your nameservers.

You can also load your website, recode the pages for the new database and test it all before it goes live.