Hi,
I suppose your emunand is on hidden partition, which means you won't be able to copy the entire contents of your SD card to the new larger SD card.
Try this and see if it works:
1). Download a SD Clone software (minitool, clone SD card, etc)
2). Clone the contents of your existing SD card to your new 256 GB card. If you did this right your 256 GB should now be an exact copy of your 128 GB card, test if everything works at this stage.
3. If step 2 is successful, then Mount the new SD card on your PC, and open any partition manager software (minitool, diskmgmt, etc) and select your new SD card, now right click and select option to 'Extend partition' to its full capacity.
4). You should now have a 256 GB Emunand without losing any content from your older version.
Hope this helps!
All the best.