toggling :colon-mode in already created packages?

[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