msi upgrade requires original package?

I just upgraded from rc5 to rc6 using the recommended options to
msiexec to perform an upgrade-type install.  The installer complained
that the rc5 msi file was not in the directory with the rc6 msi file.
Once I copied it there everything went fine.

I was under the impression that the rc5 information was cached by
Windows somewhere and the source msi was no longer needed.  I guess
this is not correct?  Am I supposed to keep all .msi files in the
same directory then for proper upgrading/downgrading?
-- 
Joe Buehler