Changing the size of a varchar field

Using PG 7.3.x, how stupid is this:

    UPDATE pg_attribute SET atttypmod=<new size + 4> WHERE ....;

I had to do this on a database, and surprisingly it seems to have
worked just fine.  The columns accept a larger value, the existing
values are still intact, and I've seen no other strange errors...

Am I living dangerously, or is this an "okay" thing to do?

eric