Hi Pilou, here is are the steps that I'm talking about.

Say you have a bunch of boxes and you want to replace some random number of them with a sphere.

So you start out with things like this:



First select all the boxes like this:



Then run the random deselection script from here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3927.2

Some random pieces will get deselected, looking something like this:



Run the script from before in this thread to place a point at the center of each selected object:



Now delete those selected objects:



And finally use Petr's CopyToPoints command () to duplicate the object:
http://kyticka.webzdarma.cz/3d/moi/#CopyToPoints



- Michael