From KLM:
You may cancel or modify a Classic Award; however a fee of € 45 will be charged.
This can be risky. The best way to do this:
a) do nothing until the Promo Award becomes bookable.
b) Check if there is a Promo Award available for the dates you want. If so, book it immediately.
c) only when your Promo Award is booked, should you request your Classic Award to be cancelled.
For this to happen, of course, you have to have enough miles in your account to book the additional award before having the miles for the Classic Award refunded.
Note that it may take some time for the Classic Award miles to be refunded, in which time availability for your Promo may disappear.
So exercise caution or you may end up having to book your Classic Award all over again - or worse, you might find only Flex Awards left!!!
Your case may be more complex, given that you have a return award. A good rule of thumb is to always book awards as two separate one-ways, where this is possible (not possible to book a one-way FB award to the US if you are not a US resident; not possible to book one-ways on all FB partner airlines). This gives you the maximum flexibility (though, of course, if you need to cancel, it does mean you have to pay the cancellation fee twice).