how to change sql server product key

According to this article (referenced originally by nopol's answer) it is possible to change the product key of a SQL Server 2008[1] installation by performing the following steps:

1. Open SQL Server Installation Centre
2. From the Maintenance screen choose Edition Upgrade
3. Enter the new new product key

It is also possible to change the product key at the command line using the command

Setup.exe /q /ACTION=editionupgrade /INSTANCENAME=<%INSTANCE_NAME% /PID=%NEW_PRODUCT_KEY> /IACCEPTSQLSERVERLICENSETERMS

Where:
%INSTANCE_NAME% is the SQL Server instance that will have its product ID changed`

%NEW_PRODUCT_KEY% represents the new product key that the will be applied to the specified SQL Server Instance.

Notes:

1. The linked article specifically references changing the license key for SQL Server 2008 Developer Edition. This author is unable to determine whether this process allows changing of keys between other editions due to lack of required software to test with.
2. The /IACCEPTSQLSERVERLICENSETERMS parameter indicates your acceptance of the SQL Server licensing terms. Make sure you understand the license terms that apply to your (or your organization) before using this switch.