We're considering using Features module for pushing out changes to production servers.
If a feature is to update an existing node with exisiting data, how will it behave?
For example, if the update will remove a field, will it remove all data corresponding to this field? If I just rename a field, will it create a new field on the node on the other server or will actually rename the field?
In general if you remove a CCK field on your local site -- you are liable to loose all your data in it. So if you move it to production, using features, the CCK field will get deleted on production and you will loose all your data there too.
Features repeats the same actions that you do on local machine on production machine. So if you simply rename the label on the field (or even the widget type on local machine) you are not going cause any data loss on local. That's because CCK allows people to change Labels and Widget types (though you cannot change the machine name anymore). Enabling the "renaming" feature on production will not cause data loss there too, just the label of the CCK type will change there.
You can perform the above as a simple experiment to verify it.
I am assuming you're not using crazy modules like http://drupal.org/project/cck_field_rename I don't know how features would behave in those scenarios.