Hi all, sorry if this has been covered elsewhere and I missed it in searching:

Looking to book a Y+ redemption ticket to JFK later this year. Right now there's good Y availability on my dates, but as we know Y+ redemptions aren't available for another couple months. If I book a Y class ticket now to make sure I have a seat on the plane, is there a fee in a few months if I'm able to pay the additional miles to "upgrade" to Y+ on the same flights? I'd feel foolish if I pass up good Y availability now for the hope of Y+ availability later only to get hosed if neither ends up being open down the road. (I am aware that changes/refunds incur a fee, but I'm wondering how strict they'd be about this if the change is to give them more miles for a higher class.)