You can build on 5.3, but If you want to use IAP and ads, it will never work on this version because Epic uses deprecated and incompatible libraries that make them unusable.
Just use 5.2, it works fine on this version.
Also, don’t convert your project to another version, it’s always broken.
Create a blank project on the version you want, then replace its “Content” folder by yours.
You’ll just have to reconfigure your Project Settings and that’s it 
Anyway, when you are asked to “rebuild from source”, open your project in visual studio (by clicking on the .sln), then on the right of the window, right click on your project name and click on “build”.
But again, it will be broken on 5.3 whatever you are going to do 