I bought a used JL 12W6v2-D4 sub a few months ago, specs below. Im wondering if I should upgrade to a new Alpine Type-R 12" 2000W Peak / 1000W RMS
Impedance 2 ohms
There is a sale going on right now at a local dealer.

Current Sub: JL 12W6v2-D4
Continuous Power Handling (RMS) 600 W
Recommended RMS Amplifier Power 200 - 600 W
Nominal Impedance (Znom) Dual 4 Ω

My Amp is a JL JX1000/1D
Specs:
JX Series 1-Channel Class D Car Amplifier
RMS Power Rating (14.4V):
4 ohms: 500 watts x 1 chan.
3 ohms: 750 watts x 1 chan.
2 ohms: 1000 watts x 1 chan.
RMS Power Rating (12.5V):
4 ohms: 500 watts x 1 chan.
3 ohms: 700 watts x 1 chan.
2 ohms: 900 watts x 1 chan.

 

Is there something you're unhappy about with the present system set up?
IMO changing the cabinet will make a bigger difference.
Whether you change the insulation, bracing, porting, cabinet design(type), those changes will be more audible than changing the brand name or impedance.
Don't be impressed by spec sheets, listen to your ears.

 

are you after more bass? deeper bass? louder bass ect?