When you have an older release - e.g. R2012b in your case - installed and then plan to install a newer release (here R2014a or R2014b) using the same license, you can simply install it in parallel to your existing installation. You do not have to do anything with the license file of the older installation.
Technically, I always recommend to work with the two releases in paralell, as typically you will not upgrade all you code and models at one point in time, but only when you need to make changes. I recommend working in different working directories!
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