The biggest difference between a grand and an upright is the "feel" of the keys because the underlying mechanics work differently. On a grand, it's a lever that sends the hammer flying straight up, and then it falls back down thanks to gravity, whereas in an upright it's more complex since the vertical movement of the keys needs to be translated to a hammer that moves horizontally and bounces back without the same assistance from gravity. Overall, the grand's mechanics tend to feel more responsive and exact. The difference is most noticeable when you play repeated notes, it's easier on grands, but it's there even in other cases. I also think you can work more finely on tone production on a grand vs an upright.
If you are satsified with your upright, I see no reason to switch. It's mostly if you want to get to a really proffessional level that practising on a grand may become important.