Increasing the length of a varchar domain

I need to increase the length of one of my domains from 16 -> 20 characters.
How wise/unwise would it be for me to issue an

UPDATE pg_type SET typtypmod = '<newlength + 4>' WHERE typname = '<domain name>';

command to achieve this. I'd prefer not to dump/reload the database. I
remember seeing a thread on this a few months back, but can't seem to find
it anymore.

PG version 7.4.2 (I know, needs updating...)

Any advice?

  - Rich Doughty