You have two tools for the grid: the Structural grid and the Grid system tool. The latter is great while and if it works (auto display in S/E for example, many options for most of the situations).
The latter is best edited after placing by selecting it with the grouping on and changing the parameters.
The former can be treated as any 2D set of lines and objects - usually, clever marquee stretching will do the trick. And redefining is not so horrible, come on
Djordje
ArchiCAD since 4.55 ... 1995
HP Omen