First and foremost, yes Spike's Tactical receivers are legal in this state as long as they comply with all other laws. Spike's Tactical receivers will continue to be legal after January 1, 2017 but importing or purchasing after that date will negate their ability to be registered under the new law as assault weapons.
Second, it is possession IN the state. It's not just a date cute off like pre/post ban. You need to be in possession of it and be in the state before January 1, 2017 to be eligible to register it as an assault weapon.
So you're good to go purchasing a Spike's Tactical lower. If you move into the state with it before January 1, 2017 it will be eligible to be registered as an assault weapon if you so choose. If you choose not to, or if you move into the state with it after January 1, 2017, than you will be limited into what you can do with it. You can configure it as a rimfire, a non-semiautomatic, or a featureless rifle (google Monsterman grip Hammerhead stock to get an idea of what that looks like if you're not familiar with it already).