Not sure how to word this properly, so I'm going to try with an example. Hopefully it makes sense...
Project A, B, and C share 4 issue types.
These 4 issue types all have unique screens/screen schemes assigned to them.
Each unique screen shares a custom field that is multi choice.
Project A wants these 4 issue types to show the multi choice custom field as a text field.
How can I change this shared custom field type for only Project A?