In your dreams... :-)
Seriously though -- you (the Board of Directors) can do anything you want with respect to vesting. But as others have noted, it may hurt your chances of future investments, even if it doesn't kill the seed round.
Perhaps a more important question to ask is why you had a 1-year cliff in the first place? Generally it's so a co-founder doesn't walk away with a significant number of shares, if they decide to quite after a few months. But whatever the reason, what about it has changed, just because you're getting funding?