Yeah. It'll be a whole lot easier if you :
a) want to move everything the same distance. Just use the move command
and select everything you want to move. I prefer to input the change in
height by hand, ie I make the origin of the move 0,0,0 and then the second
point is 0,0,distance where I can just put in the z coordinate.
b) want to move everything to the same elevation. You can just select
everything then use the property box (just input the elevation you want).
This is one of those things where there's "a million ways to skin a cat".
I'm sure lots of other people would do it a lot of different ways...