Tesla has said Model 3 has three general assembly lines, so I can't see GA2 being for anything but the Model 3:
But, sure, it may be shut down for updates, and I'm certainly not saying all the capex was wasted. It wouldn't surprise me if some of the machinery is now in the tent, for one thing. But it's always more expensive to redo something than to get things right the first time around.
At this point I strongly suspect GA2 is being rebuilt. Here's my best guess:
1 -- they built GA2 and found a lot of things which didn't work
2 -- they built GA3, learning from GA2, and found a few things which didn't work
3 -- they built GA4 while frantically rearranging GA3 and GA2 without interrupting production too much
4 -- they got GA3 + GA4 up to the target 5000/week (apparently 3000 GA3 and 2000 GA4), so they can shut down GA2
5 -- they are now retooling GA2 to try to get it running at a full 5000/week as originally hoped, or at least 4000/week
6 -- then they'll shutdown GA3 to bring it up to GA2 speed, at which point they'll hit 10K/week in GA, hopefully without GA4, but with GA4 if necessary
This all costs money but I suspect not that much; most of the equipment will be fine though a lot will have to be moved from place to place.
In terms of bottlenecks for 10K, I think the paint shop and the body line (at last report, struggling to stay above 700/day reliably) will be the most expensive and longest-lead-time issues.
-- They may need a second body line, but I think they'll try to avoid it, and when they fail to double the body line speed (they'll improve it, but quite possibly fail to double it) it'll mean a delay in 10K as they won't start building the second body line until later than they should have.
-- They may need another paint line or two (for reference, it is clear emissions regs are not a problem). At some point you can't speed up paint any more, and I hope Tesla will recognize when they're reaching that point. Paint lines are slow to build, so they better get started now.. I figure Tesla will go with electric heating to bake the paint on the new line.
-- The battery pack lines at the Gigafactory will probably just be flat out duplicated as needed, and will go quicker than building a new paint line. And cheaper now that the lines have been "debugged".