You can register at any time you want, but if you don't use the option you mentioned "I'm a registered user and my registration key has expired, I need a new one" and you use the same email address as before, then it will send you a copy of your existing key.
So use a different email address and it doesn't matter what registration option you select 1, 2 or 3 you will get a new different key, valid from that point 14 months from the 1st day if the next month.
Use the "I'm a registered user and my registration key has expired, I need a new one" option and regardless of email you will get a new key. It doesn't matter when your existing key expires as this isn't even considered for this option. But you don't even need to go down this path until January 2009 and avast would start to remind you 3 weeks before (I believe).