Bug25882, upgrade from non-Debian has gid conflict
This sounds like a policy decision. Either we
a) add a script to detect old passwords, if this is possible since
the user probably just copied the password file,
b) write a script for taking an old password file and coercing the
filesystem into the proper id space. BTW, I have done this by
hand and it isn't too bad. I simply added 1000 to the old IDs.
c) we document this with a warning for now and revisit it with one
of the above solutions.
I'd say go for the last since this has been sitting since the middle