Below some indications, if that can help some people.
(based on my own configuration (XP) My menus are french, maybe my english translation is not exactly the one you'll have).
1) Enter Editors within ManiPlanet TM game
2) Load a map
=> Game shows folder "My Maps\"
3) Click second icon (Folders)
=> That opens the folder in Windows. For me it's "D:\Documents and Settings\MyAccount\Mes documents\ManiaPlanet\Maps\My Maps"
==> This is where ManiaPlanet searches for the maps to encode, I imagine.
4) Come back in the Editors. Under the main folder (My Maps\) the TM1 maps you loaded here, appear.(knowing that, I try a last thing :
cd /d "d:\Documents and Settings\MyAccount\Mes documents\ManiaPlanet\Maps\My Maps\"
"C:\Program Files\ManiaPlanet\maniaplanet" /computeallshadows /collections=Stadium /Lm Quality=High /useronly
-> But that always does not work)
5) Select one
6) Click Edit (If there is a password, you won't be able to edit)

7) Just click 2nd icon like a floppy disk to Save the map
8) Message "You have to relaculate shadows before to play.." : Click OK
9) Choose a quality : (very fast is Disabled) / Fast / Default / High
=> A processing gauge moves .. wait until it's complete.
10) Then you can Save the map
11) A summary screen displays infos upon the map. You see original author name is not changed. Nor the validation, that it remains

12) You're back to map editor, you just have to exit. (First icon, like a Left arrow < )
13) Go back in Windows folder
=> Processed map is named "$f0fdrop down.Challenge.Gbx", and has not been replaced. A new file has been created : "drop down.Map.Gbx". However, the color ($f0f = Pink) still displayed in TM². The big difference is the size of the map : it goes from 2Kb to 260Kb ! (An other goes from 2Kb to 161).
14) In map Editors.. in the list of maps, "drop down" now appears twice. (Old + new one).
With those informations, I'll build a "Super Macro"
But before, I gonna try to encode my 1370 maps on my laptop with Win8.. maybe that wil work.
If somebody know where the problem is ... or at less how to go through the password problem ?
Thx,
ccx