Because QCad isn't parametric, and therefore lacks constraints, the only way to achieve want you want is to re-draw your dimensions. There may be a way to do it by re-scaling them, but I haven't tried it. (FWIW, I tend to put all dimensions on a separate layer). You can also change the position of the number above the line by highlighting it and then dragging, holding the light blue highlighted point- if that makes sense?
I'm not to sure about your second question though. There are ways of manipulating entities using the move and copy tools, like changing layers and stuff. Perhaps Andrew can shed some more light on that?