wow 3 years later but could be useful. Its not really necessary to reauthorize with the new serial number until you run out of days on your temp license, but if you are antsy and want to for some reason authorize with the new serial number, you can force a challenge code. Here's how you do it:
Launch Max/MSP with the Control and Command keys held down (Control+Shift+"c" for PC). This will force open an authorization dialog.