Probably the most surprising of the non-upgradable attributes of Custom Fields is the list of picklist values. These are completely modifiable by end users; they can be added, modified, or deleted. This means that you need to be very careful about how you reference these in your Apex code, and consequently, you should provide guidelines for your end users about editing and applying changes post-upgrade.
The Salesforce Translation Workbench tool, under the Setup menu, can be used to effectively re-label a picklist value by editing the user's current language without having to rename its value on the field definition. This can be done after the installation of the tool by the end user admin.
Also, remember that the platform treats picklist fields mostly as if they were text fields; as such, you can query and even update a picklist field with any value you like. Keep in mind that you can upgrade customer data (if you change or delete a picklist value) through an Apex post-install script.