Hi,

I have an issue to move several nodes of my shape in order to make it shorter. So I want to keep rounded corner with the same radius but make my button thicker.

I select all needed nodes pressing SHIFT + N (edit path) selected. However when I try to move nodes, it destroys my shape and I can not make it shorter.

and once I want to move them this is what I get:

Have you tried Path->Outset/Inset? No clue what you mean by "shape shorter but button thicker".

Oh - and I recommend the use of just 3 nodes for half a button.

yes, this shape is done by Outset and inset.

I meant by "thicker button" it's to build a 120 px wide button instead of 462 px wide that I have right now.

I don´t see the problem here. I´d select then all nodes from one end and either click&drag (cursor needs to grab one node - not the stroke) or use arrow keys (with shift for higher values) to move horizontally.

If you want to change the size - while keeping the midpoints as midpoints - you will need to delete all of the left side nodes for the entire asset.  Do this by deleting a segment between two nodes on each object in the overall asset.  I would delete the segment just to the left of the midpoint on each object.  Then box select all of the nodes on the left side and delete them.  This should leave all midpoint nodes and right side intact.  Box select all of the nodes on right side.  Select a node - hold ctrl down - slide to left until size you need.  Remember, this will be half of the final asset.  Ctrl d your selection.  Flip selected objects horiz.  Turn on snaps.  Move left half until midpoint nodes snap to each other.  Select each pair of midpoint nodes and join them.  This method of work will produce a symmetric end result - just moving nodes will not.

The shape change is likely due to the nodes on the long sides being curves. The curves can be changed to straight line segments by adding nodes and converting.

@alainroger The reason for the shape change is a duplicated or errant node.  When you tried to relocate the others, it served as a pivot to cause the kink in your line.  Look at your second image.  Do you see that small black sliver between the yellow and the green?  It points to the bad node.  Also notice your corner nodes top and bot are not vertically aligned. 

Make copy - delete green object - hunt for copied or overlapping node - delete it or join it to its copy - rebuild line - then do as I suggested earlier.