I don't think it need be an either/or situation. You could send a neutral sort of card and think of it as a "place holder" for the potential future relationship. The "enjoy your special day" sort of card rather than the "Thank you for being my Mom" sort of card. I found it was important in hindsight not to have created opportunities for new pain for me or my parents while I was "in transition." Neutrality was a better option for me. Same with birthdays, etc.