Sorry I may have mixed it up, when you have regular projects you ahve this field in the general tab of the project settings and if you have VS 2015 and 2017 installed you get different options there. After upgrading to VS2017 I had to use some tool to remove the remaining toolchaing from 2015 completely, my programs used a mix of old and new.
I don’t know how this can be set for Unreal Projects I seem to remember there was a way but I don’t know anymore. For me, the final solution after many tries was to get rid of Visual Studio 2017 and even restore windows to a previous point in time before I installed 2017.
Conclusion for me: As long as Epic does not request VS 2017 as mandatory, I stick with 2015. There isn’t anything I need in 2017 anyway.
Oh and BTW: My Visual Assist performed terribly in the VS2017 environment, that was another reason to go back to the older one
