You won't be able to do it in advance.  You have to be at the computer when
you call.

I don't think most people understand the process accurately.  They aren't
monitoring your computer every minute and you don't really "deactivate" the
old number. You are essentially just associating the number with new
hardware.
It is just when you try to activate a product, it checks to see if it was
activated previously.  If it was, then you need to call them (India
probably, who you can barely understand and they won't understand you).
They essentially transfer the key to the new configuration.