At the end of 30 days, your computer won't fully boot to Windows. It will just boot to the login screen and not allow you to go any farther until it is activated (which it will allow you to do).
You should be able to activate even though it has been activated before, provided you are only using the OS on one computer. I have done it successfully. You will have to activate over the phone. When it doesn't allow you to activate with the automated system, it will send you to a live representative. They will ask you to explain your situation. You will tell them that you are doing a complete system upgrade and you are only using the OS on this one computer. They will give you the code, and it will work. I've done it before more than once. If you are using the OS on more than one computer, then you will risk the wrath of Microsoft.
ps If you tell them that this is the ONLY computer that you have ever used this disk on, they will know that you are lying, and they may not help you. Your COA number is already on file and they know it has been activated before.