Wouldn't call a transmission a complete update to a model. SVT was absorbed into Ford Performance along with the RS division, so essentially it's all the same. I can't recall one singe model (be it SVT, Ford Performance, whatever) that was built for one model year just prior to a major model change.
If you think about it, a 2018 and 2019 GT500 would be much like the 2013/2014 before the last change.
What are the current chances of a 2018 GT500? Slim to none.
I doubt Ford will release a 2018 GT350 -and- a 2018 GT500.