On past cards it hasn't worked like that - you can populate/un-populate or even upgrade the VRAM and the GPU just works detecting the correct amount of VRAM available (up to a point). Unless something has changed you don't need to mess about with firmware locking.
Though wouldn't be the first time shipping a GPU with more memory populated than is useable would be a little dubious though it has been done in the past to mask production issues.
Though wouldn't be the first time shipping a GPU with more memory populated than is useable would be a little dubious though it has been done in the past to mask production issues.