When you work on a feature that you create an ISV license for, you will at some point apply the license to DEV and TEST boxes. 

If you have an error in your licensing artifacts or you need to change something about the license it is however not possible to update license by importing a new license for the license key in question.

One example of an error could be that you create your license key without attaching the resource holding the public key.

To reload the license you have to find the corresponding record in SysConfig on the database and delete that record. This is not a trivial task. 

So, of you try to load an existing license it should overwrite what you already have.