toggling :colon-mode in already created packages?

[gooch@TIJERAS.SW-SW.Dialnet.Symbolics.COM (William D. Gooch)]

    Date: Sun, 15 Dec 1991 19:27 CST
    From: starnet!bass!lakin@apple.com (Fred Lakin)

    I still have a lot of old data files with single colon pkg prefixes;
    I know in advance when I am about to read one. 

    What I would like is to toggle the :colon-mode in the relevent pkgs to
    T just to read in the files, then toggle back to nil so the files can
    be written out with modern colon syntax.

    Is this possible without destroying the pkgs? rebuilding 'em while 
    still afloat so to speak ...

    tnx,
    -f

I think you can safely do this by using 
(in-package "<package-name>" :colon-mode :internal) and
(in-package "<package-name>" :colon-mode :external)