Here you go just a brief info about your Q..
Dial-up..
Dial-up access is a form of Internet access via telephone line. The client uses a modem connected to a computer and a telephone line to dial into an Internet service provider's (ISP) node to establish a modem-to-modem link, which is then routed to the Internet.
Broadband...
A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings. The defining characteristics of LANs, in contrast to Wide area networks (WANs), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines.
It means you do not need to remove the dailup connection and your new Broadband will be install, just you need to do is remove the dailup network icon , or if you wane use for stand by you can keep it too..
few ppl use broadband and keep dailup for backup..
GOOD LUCk.