It may cause a conflict, or at least a minor issue (the domain might be updated with the new IP, where there's no site at that time).
Unless you sign for a 3 year deal and want to move that domain over to us and get it extended (you get a free domain with the 3 year deal), you can sign up using one of the sub domains we offer for free.
Once you have activated the premium account, all you need to do in order to transfer the free account you currently have is to submit a support ticket through the clients area. Don't forget to include your current cPanel user name.
We will then move your current account, including domain information, over to the premium server. So in the end you have the same account, just different server and package.