<<Another method is to select the bottom cap and inside wall surfaces, and use Edit/Separate to break them out into a separate object. Now you can use Transform / Scale / Scale1D to scale the height of that fragment, pick a point on the top level as the origin for the scale. When you are done, select that inside piece and the outside piece and use Edit/Join to glue them back together into a solid.>>

Yes thats the solution i find too as a good one ;) : [Separate, scale 1d, join] is a safe solution (as adj. faces are concerned too).
Thanks all for help! now i return to MOI adventure... tadaaa...