You should have gotten (will get) a letter with CORES (COmmission REgistration System) that will have your password. If you do not have that, you can download the CORES Registration Update/Change For (FCC Form 161) from the FCC homepage http;//www.fcc.gov/formpage.html (it also can be requested by phone or fax). If your forget your password you will have to contact, you will have to contact the CORES Administrator, and provide them with your PIN, taxpayer ID number, along with your FRN, to obtain the password hint. They can only give you password hint, or reset your password. For security reasons new passwords are mailed to your official address of record. If you have any questions call the CORES ADMI 877-480-3201. (The following was taken from my CORES Letter).
I had run into a problem when they first issued my License -- they had my address wrong -- so I had to get mine changed even before I got any of my "Official" mailings. I was able to reset my password online, and if I recall it was not all that hard. You do not need to get a new FRN, but simply reset your password.
There is also another option on getting a new call sign, and that is going through a service, like W5YI, it does cost something like $50, while the FCC is (whatever it is now). My father went though W5YI, while I went through the FCC. When I did I had my password written down, and it was not that hard. So if you do want to go through the FCC, all you need to do is either call them, (or email) and get them to reset your password, once you get in, I would change it, write it down, and you should have no other problems.
One hint about changing your call, make sure 1) the call you want is valid, 2) that you have the right group for your license class, 3) that it conforms with the license structure. I used to get a laugh at one website that used to list the "rejected" calls, where one time someone had requested NCC1701 for a call, or VA3DER, while they look cool, neither are valid. That or the ones would put "ANY" (as in any 1x2, or 2x1). Have fun, and good luck.