While it's a thought in many peoples minds, I like to think it won't happen.
Space mountains ride capacity looks to be 1800-2000 people per hour which is one of the higher ones in the MK. Considering the ride is almost never a walk on, then it's doing the job it is supposed to do and is still a major draw for guests.
If they are trying to grow overall capacity, then it would be counterproductive to toss such a large people eating attraction out. Space mountain is a proven attraction with solid throughput and guest demand in the Magic Kingdom. If they were planning to retrack the ride back in 2010, then lots of the prework has already been done and paid for. Things like track replacement strategy, building evaluation, track design work and cost estimates would have already been done. I think they are just spreading the cost out over a few years. Once Tron is up and running Space Mountain will go dark for a bit while they refresh the track. Who knows, maybe can get some animated props while they are in there too
Here is to hoping we see space mountain go another 44+ years with Tron right next to it