Postponing our date

I think just be honest with your reasoning so people know not to worry

Postpone the Dates.
Same idea as the Save the Dates.
A cute picture of the two of you with the heading "Postpone the Date"
Maybe some sort of vague explanation like:
"We're still madly in love, but we need to postpone. We can't wait to tell you our new date, once we figure out when that is."

I like the cute postpone the date idea

Send some sort of "date change" card.

How many save the dates did you send out? If possible, I would call everyone and let them know. I would hate for someone to request off work or book a flight. A phone call would give you piece of mind that everyone knows first hand from you. This also will help the rumor mill if anyone were to get the wrong idea from a postponed card.

View Quoted Comment This is super cute!

Do you know the new date yet or will you know pretty quickly? I would just re-send save the dates with the new date and make a special note so that people know it’s a new date and to throw out the old one. But like the pp said you would have to do that quickly so that no one makes travel arrangements or takes off work