Problem of data merging in my 3DSs

[AIAS]

I wonder whether anyone can help. I moved all the data succesfully from my original 3DS to a 3DSXL earlier in the year. Howver, my 3DS slider malfunctioned. Enjoyed the games but missed the 3D perspective so decided to send my XL back for repair. In the mean time and with Pokemon X and other supa games around couldn't stop playing so loaded loads of data onto the original 3DS.

 

My XL is back repaired so ideally I would wish to merge all data into a bigger SD card. Tried once but failed....

 

Has anyone done that succesfully? I have 2 Miis already on 2 different systems. Lost my friend list. Feel like schizophrenic...