If you have data on the smaller card, you will need to save that data, either to the phone itself or to a computer, before you remove it and insert the new card.
The fastest way, which also allows you to save backups, is to leave the original SD card in the phone and connect the phone to a computer via USB cord. Then move all the files from the card to an empty folder on the computer. Once done, remove the phone and do your card replacement. Once you have done that, and restart the phone, reconnect it to the computer. You should now be able to copy all the files back over to the phones new card.