I'm not one to answer, but check the directory (in your Home directory) that Mathematica uses to store your prefereces and also .m in Mathematica's program directory. (.m is the "simple text" version of .nb, you can say)
The choice very well may be in one of those .m, and if it is: you can edit it.
An alternate strategy (other than waiting for WR to fix it, which i assume you already requested by support email), would be to trick it: in your program folders or "program links" make 2015 to run where 2017 is found, tricking it to run 2015 instead of 2017. You'd then need a different "program link" if you were starting 2017 for some other purpose.
