Running windows 7 Ultimate a tech change My windows 7 Home Premium for the 7 Ultimate

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I need to buy a product key for windows 7 home premium some "tech" change it for windows 7 ultimate. And my key is not working any more, is there on other way?

 

[TrainableMan]

If you still have your Home Premium key you can reinstall. It's a bit of work but it would be real. I would not trust any W7 Ultimate key you might buy online unless you buy it straight from Microsoft and that is expensive.