Assuming you upgraded the license keys and are now using both keys in your environment: the old license will still "work" but it is 100% no longer a valid license and potentially exposes you to license violation. Of course I would expect there to be some understanding for a migration period, but this is reliant on goodwill of VMware nothing else so keep it a realistically short period at most 
This is not legal advise or suggestion, just writing it as I see it.