Wanted to try it so I bought the yearly and within a couple of days decided it was good enough to get a lifetime. You should offer a discount or credit toward lifetime to those who buy the annual and soon thereafter decide to upgrade. Either a prorated credit of the yearly fee, or a 20% discount on the lifetime.
It’s good for you as well as your users, because they’ll be more enthusiastic in the first month and likely to buy; whereas they may lose interest before the year is up. Otherwise, a user who has bought a one year subscription will feel compelled to wait until the year is up before buying another year (or a lifetime), because they don’t want to feel like they’ve wasted the ne year purchase.