It sounds like what you're actually wanting is renewing into a new plan, but such functionality doesn't exist. When you upgrade to a plan it's completely replacing the old one with a new plan.

I will run it by my boss and see if he's okay with people skipping a year's payment if they register in November-December.

I don't think I'd word it like that, lol. They're skipping 1 months worth of value. It benefits the customer more than anything as they don't feel cheated by paying 12mos for only 1mo of service. Would you be happy with such a purchase? Personally I'd issue a charge back and never come back, lol.

You may want to consider using 1 year durations in that case instead of by calendar year durations. As with by calendar year durations if they subscribed at the beginning of December and it ends in December their subscription will expire at the end of the month. The only way to avoid that is to use the parameter described above, which will subscribe them for next year and give them December free. However, your usage doesn't sound like a typical subscribe for a service so I'm not sure what to suggest.