AutoCAD 2009 makes it easy to make changes to clipped xrefs or blocks.
XCLIPFRAME controls the visibility of xref clipping boundaries. It is 0 as default and not visible. If you set it to 1 it is visible and you can grab the grips, stretch them and make changes to the boundary. The problem is that if you plot the drawing the boundary also will plot.
A workaround to have it visible but not plotted is to make use of named plot styles (STB). Create one that is invisible by setting Screening to 0. Then use this plot style on the clipped xref object. If you use color dependent plot styles the closest you can come to a workaround is to assign a dotted linetype and set the linetype scale so high that only a few dots shows.
Wish for AutoCAD 2010 is to add XCLIPFRAME=2 that is visible but does not plot. Another wish is to be able to add and remove vertex points on the xref clipping boundary. If you need to make these changes you now have to redo the boundary either by creating it from scratch or generate a polyline and edit it and use it for the new boundary.
If you want to clean out a drawing from objects that are outside the boundary XClipClean is a software that delete and trim to the boundary automatically.