Easiest way to upgrade a generator gem in my app?

I was wondering if anyone could tell me the easiest way to upgrade a
gem that is a generator?

In my particular case, I'm using Auth_generator 1.5.3 and its great.
Now I see there is 2.0.1 with some new features. Now I've already ran
the generator and created all the auth files and integrated it into my
system and made a few modifications.

If I do: gem install
http://penso.info/tmp/auth_generator/auth_generator-2.0.1.gem

Can I just rerun the generator: ./script/generate <my auth controllers>
and have it upgrade my auth controllers/models/tests with any new
features that are in this release and leave my custom modified code
alone?

Or will I have to just rerun the generator and redo any changes I've
made to my previous files? I basically just want to upgrade with as
little breakage to my apps as possible. Thanks for any advice.

Many of the generators are not what you'd call "update safe" (although
I have no idea about this particular generator).

Probably the safest, albeit a bit convoluted method, is to update a
copy of your application and then use a file merge utility (if you're
on os x, apple's FileMerge.app is nice) to merge their changes with
yours.