How much game scripting and kismet is involved?
For the gameplay code, it’s going to have to be redone. We ported from UDK to UE4. For us it didn’t take especially long, the art was faster than the gameplay code as it was mostly just resizing and importing. But, we did not have complicated environments. I don’t think it’s possible to say it will take a month or any time in particular. You may be able to do it over the weekend or it may take a long time.