Couldn't find documentation showing that upgrade from SQL 2014 evaluation version was possible to developer edition. I just successfully converted an evaluation version to developer edition.
- Obtain the key for the developer edition (in my case I had to download the .ISO from MSDN downloads, and go through the installation wizard to get it)
- Run the installation center app for SQL Server 2014
- Select edition upgrade
- Plug in your new serial from the developer edition. Pretty simply, but thought I'd post a confirmation for anyone wanting confirmation the upgrade path was an option for developer. Probably somewhere in the MSDN documentation, but I couldn't find it with a quick search.