Currently, there is no good way for you to change the orientation of the card online, unless the design has another variation. However, if you request free designer assistance after customizing your proof, you can have our designers do this for you on our end. Just select "Request free designer assistance" at the end of the customization process and let us know you'd like the card flipped. Our designers will make that change and send you back a proof for approval.
In most cases there should be no problem changing the orientation, just keep in mind that if a design depends on the card being a certain orientation, we may not be able to rotate the card and keep the design the same. Also, we'll need to keep the card size the same. If you want to change the card size, please see: Changing card size