@OP First off lets clear some myths here, the engineering department of LUMS is at par with NUST's engineering Deprt in terms of employment prospects and Academic standard if not better.
Now coming back to your question... Yes! it is completely normal to change field when applying to universities. At first it may be difficult for you to cope up with basic concepts of your respective subjects but eventually you would settle in. Also, most of the foundation stuff of all subjects is repeated in universities (albeit at faster pace) so it won't be much of a problem.
But my question remains why are you applying for management sciences, is it because you are more interested in people management than engineering itself or is it because you want to apply to LUMS (but had the preconception of an inferior engineering deprt)?