You can do that without causing problems. The ammeter will no longer read correctly. There'll still be some current running through it, but nowhere near as much as before.
There's nothing wrong with the idea MAD discusses, but (as with other stuff on their site) their method has problems. You might want to read suggestions and discussion
here and
here.
(Also, people have successfully bought a cheap Sunpro voltmeter, torn it open,
and mounted the old ammeter face over it with excellent, clean
results. See
here and
here.)
