I think it's time to fess up and let her have a good long cry or two - and sooner is better than later. You are a human, and have human limitations and frailities - such as not being able to anticipate a wish list change.
I think she will be better able to handle the truth than to think Santa is dis-ing her.
Easy for me to say, as we never did the Santa thing. DS, at 3, used to cry himself to sleep about 'all the children who have parents who lie to them.'
OTOH - those tightfaced complaints from the daycare teachers about DS spoiling other kids fun weren't fun.
I feel so much compassion for you to see DD headed for disapointment.
Grinity