'rpm --freshen': dpkg Equivalent?

[Kenneth Jacker <khj@be.cs.appstate.edu>]

How can I use 'dpkg' to install a new .deb, but *only* if the package
is already on a machine?  With RPM, you can use the "freshen" option
("-F" or "--freshen").  But how with 'dpkg'?

In other words:

  o If the package is already installed, then update it

  o If the package is not installed, do nothing (do *not* install it)


Sorry if this is obvious, but I can't seem to find out how to do it!

Thanks,
-- 
Prof Kenneth H Jacker       khj@cs.appstate.edu
Computer Science Dept       www.cs.appstate.edu/~khj
Appalachian State Univ
Boone, NC  28608  USA