Give Disk Utility a go (it's under Applications > Utilities)
Select the volume to copy (it'll be listed under the SD card)
Click New Image & pick a location to save it to.
Once the image has been created eject the original SD card & insert the new one.
Click Restore, select the saved image & then drag the destination disk from the list on the left - choose carefully (make sure it's the new SD card) as it will overwrite irreversibly.
Did I mention being careful? Caveat emptor.