KDE4 and re-using kde3.5 user areas

[Mike]

Although I am currently running KDE4.04 on F9 in which I allowed the user
area to be created from scratch during install (or using useradd) after
the install, I have a question as part of advance planning for when I 
upgrade other systems which have pre-existing user areas in F8.

Does anyone have any experience in installing F9, and then copying back
pre-existing user directories from their original F8 files.

What I mean by this is that the F9 install is done without creating
any users apart from root.  Then the appropriate user lines are added
back into passwd, group, shadow and gshadow in /etc and then the 
pre-existing files from /home/user1 /home/user2 and so on are copied
back into /home from backup.  If this is done and then one of these users
logs in to KDE4.x using only pre-existing files that were from KDE3.5
in the earlier system, then 
a) will the user be able to successfully login to KDE4.0x?
b) if the user can login will there be any nasty consequences from
the fact that the kde config files relate to the previous 3.5 files?
c) If there are nasty consequences then which files should be changed
or deleted before the user logs in?
d) If it is mostly OK but there are a few minor watchpoints then what
little things may bite, and how do you fix them?

In addition are there any trick needed for getting ssh-add to run in F9?
I tried adding a file into .kde/Autostart/ which executed /usr/bin/ssh-add
but it did not seem to execute when during kde4 login

Finally although for me this is hypothetical, has anyone actually 
done any of this successfully?

It is possible of course to do:
mv /home/user1 /home/user1.old
remove the user1 lines from passwd, group, shadow and gshadow
and then do
useradd user1 -c "Real Name"

then rsync -av /home/user1.prev/directory_to-restore /home/user1/
for all the directories you wish to get back, but leaving any .kde related
directories alone ....

But this is tedious - hence my original question.

Any advice appreciated for the transition of vital systems from F8 to F9
where KDE is involved.

Thanks.