Hi there, we've implemented QR Codes for retailers and can hopefully shed some light...

QR Codes are merely a 2 dimensional representation of a set of characters, typically a URL (It really is just a complex barcode that can represent letters, numbers and other characters). For each URL there can be an individual QR Code, so there is no chance of anyone creating one that duplicates/conflicts with yours... Or more accurately put, anyone can generate any QR Code and make it available; but there's no point sending people off to anywhere but when you want them to go (its like me advertising a competitors URL somewhere).

QR codes do not have a shelf-life, once they are created they can exist all the time. However the destination URL may change, so you need to ensure you do not move the page you want to take them to.
Note: a lot of QR Codes are sent to a URL shortening service (e.g. Bit.ly) before being redirected to their true location. This has several benefits:
1. You can quickly track users using the shortening service analytics (hugely beneficial if you can't get access to your own website analytics in a hurry)
2. You can change the page you want to redirect them to if required
3. The shorter the URL, the less complex the QR Code (which is very useful if you are putting the QR Code: behind glass, at a distance, making it smaller to fit onto something, etc.)

Altering the QR Code is a process of trial and error to see what affects it. The general advice I have is to try not to affect the ‘dots’ too much, but they don’t necessarily have to be just black on white either. Basically… the more you try to do to a QR Code, the less likely it can be read by all readers (and some are better than others).

If in doubt, contact me directly