Hi Bob,
actually, i did an upgrade install (2.0.4 pl2) locally and on the former distant server to have a try at it and everything worked fine except for that little lexicon problem with tinyMCE links pop up.
About the path, don’t worry, i always save the config files and htaccess in a directory not to loose the changes when upgrading or transfering and i restore them or the needed changes after every upgrade.
i then transfered the whole "solution", db and files on the new distant server without making any changes (except, of course, for the config file)
but, funny enough, i was back here to say i’d found the trick ! it comes from the pref_workspaces table that stores Default MODx workspace, something more to change when swapping hosters and that i couldn’t find anywhere in the system tab items, neither in namespaces not in params (the first place i went when running into that wrong path error).
How i found it out ? i simply made a dump of my whole db and searched for the guilty string in the sql file. I should have thought about that before as i knew it was a storing issue...

I thought that everything was stored as showed in the manager {core_path} but it’s not the case for this peculiar string in the db.
Now, i can upgrade my packages without any problem, great isn’t it ?
I still just had to solve that tinyMCE issue. Strange as it may seem, the package manager said that i had the 4.1.2 installed (the first package upgrade that failed many times before i went here and ask the question) which is said to solve that modxlink issue.
Well, i tried an upgrade locally and it worked, except for some lang files, and the link pop up was clean again.
I then went and see on the distant server why it could not find the transport package to uninstall it... quite normal, it wasn’t there !
I really wonder how it could say that the release was installed without having even uploaded the package (a little bug, isn’t it ?

). Of course, is it necessary to say, each time I try something that fails, or even just sounds curious, i clear the cache and try again before going any further.
Well, let’s use a brutal solution, i’ve uploaded the package from my local server, uninstalled tiny thanks to that new package, reinstalled it but i’ve still got that modxlink lexcion issue.
Nevermind, as it works locally, i’ll find where the problem is.
Thanks for taking time to answer and let’s hope this little misadventure may help others, at least with this workspace string in the DB...