How to update SQL Server 2012 from Evalutation to Developer edition? During the installation of a new development infrastructure, I had to install many SQL Server instances on different machines. Some were of course dedicated for production and some other for development. As we did not had licence at the moment, I installed everything with the Evaluation edition as it is written in the MSDN documentation that the Evaluation edition can be upgraded to any other version : SQL Server Edition Upgrades

I have no problem upgrading to the Business Intelligence edition as we bought the licence keys from Microsoft, but the issue is that I have no idea how I can upgrade the remaining instance to Developer edition as there is no way to get a licence key. Even on the MSDN Subscription website it is written that it does not require any licence key to be installed. The documentation does give any clue either about this issue.

So where can I get a valid product key to upgrade from Evaluation to Developer edition as stated in the MSDN documentation?

Thanks.
You will need to re-download from your MSDN subscription.  The license keys are built into the ISO files.  When you do the installation you can selecte the edition and it will auto-populate the license key.