Curator says that for everyone, it's just a warning. Other people have changed their card post-approval, and gotten it re-approved. Just shoot curator a PM after you make the changes, just make sure you have everything correct so that it's re-approved promptly.
Pretty much this. If you change the mechanic of the card completely to something different, but the table formatting and organization is clean, then a Curator will approve of it. Otherwise, if you have glaring mistakes (horrendous grammar and spelling, properly formatted table sizes/card image resolutions, weird symbols or icons that appeared out of nowhere in your 'Art By' section, etc.), then the card will be unapproved until you fix the changes.
Feel free to change it.
The Curators will be as quick as they can to respond to any changes your card idea presents.