Here are some points.
did you try to create a new webkey account?
If this is not possible contact your local Siemens contact for this.
As a minor change to Ruud's procedure:
Assuming you have a customized environment...
I would suggest to make a COPY of your current environment, and point NX12 at that.
All you need is for someone to save the File -> New template parts in NX12, and no one would be able to create a new part in your current version. (or any of a number of other "gotchas" that are of a similar nature)
Do you use Teamcenter?
If so, you need to figure out a compatible version of TC to upgrade to.
Production: NX10.0.3.5 MP16/TC11.2
I'd rather be e-steamed than e-diseaseled