Go ahead and load the new license file as soon as you get it.
There is not a "Start" date for a license, there is only an Expiration date.
so your new license should install on top of your old license without a problem.
Also by trying to install now you can find out if the new one loads successfully.
If for some strange reason it doesn't, then you still have the old license which gives you some time to correct the problem.
