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To keep your files accessible, you need to copy them to the new card.
The quickest way to do this is to put the old card into a card reader attached to your computer,
copy the files from the old card to the computer,
swap the new card into the card reader,
copy the files to the new card,
insert the new card into the phone.
If you don't have a card reader, you can directly connect the phone to the computer using the charging cable and copy the files that way.