I currently have a Nextel 7100i. I am updating today to a 8830 with Verizon. I have the Desktop Manager on my desktop. Am I better off uninstalling my current desktop manager and installing the new CD with the phone or is there an option to "overwrite" what is currently there???

Yes, you are. I don't know what is on your CD, but download this version 4.2, Service Pack 2. It has the Roxio Media package with it.
BlackBerry Desktop Manager Downloads - BlackBerryFAQ

Good luck.