To answer your tax credit question, the tax credit only applies to new cars that you purchase. If you lease the car, the leasing company gets the tax credit, and it's up to them whether you get any discount or not. early volt buyers might have gotten maybe $2-3K off because of the tax credit.
I plan to upgrade from my volt to something really nice soon, but I'm still wavering about what the next vehicle will be. A lot will do with the timing. If I need a car sooner, it will probably be a used ELR. If I get the car in 2-3 years, then it might be a model S or CT6 PHEV or whatever else hits the marketplace and touches my fancy.
One last note, trading in a 2016 will mean you will take a shellacking on depreciation. Seriously consider driving this vehicle into the ground and upgrading when the volt is on its last legs. Your pocketbook will thank you. Of course, if you are already filthy rich, then do whatever floats your boat.
When choosing a Tesla, I'm on the two extremes. Part of me would order the S60D to get the cheapest car that has AWD and no other options (maybe adding autopilot later). It would be fine for my commute, but might be a stretch to take out of town. The other part of me wants to S100D (without Performance) so I have the longest battery life and don't regret shortchanging myself. But then I'd be tempted to add Autopilot, dual chargers, adjustable shocks, and all of the sudden this just under 6 digit car hits 6 digits. I don't want Ludicrous mode as that's a formula for disaster (too much temptation to do something stupid and hurt myself or someone). If there's an option for grandpa mode, I want it. Maybe by the time I need to decide, they'll have an S110D or S120D. Better yet, how about an S200D.