Hi, I have just done it
And you know what!?
1. It is crashed. KDE doesn't start, WindowMaker doesn't start etc.. only failsafe and IceWM do but with lots of problems. Summary: applications do not start.
2. The user which I have deleted have stacked somewhere in the system. Namely, when the Lilo login comes up there is that non-existent user's login preselected!
3. The names of the users which show up in the Lilo login window are not the ones I just created, but some of the users which had been on the system two weeks ago. I have no idea where that comes from, because there are no their home folders whatsoever left.
I first checked that everything works under the new user before I deleted the old one. Indeed, it worked well.
Apparently my mistake was that I deleted an old user being logged-in as that user, but, anyway you do user-deletion as root. So, what's up?
Well all is backed up - no loss of data, but, OK, what's up? I feel like reinstalling a whole system is just easier, isn't it?
Well, well, pazienza...