It's probably similiar to this.
You can backup the system titles' saves + extdata with Checkpoint and/or JKSM. For system tittles to appear on Checkpoint, edit the
sdmc:/3ds/Checkpoint/config.json like in that example. Backup the saves + extdata on the n2DSXL, copy the save manager's folder from one to the other's SD card, and then restore the collection of Miis.
There's also dumping all the individual Friend Codes with
seedstarter (lazy way of recording without having to write down with pen(cil) and paper), and manually inviting them back with
Register Friend.
Lastly, there is a way to perform an unauthorized / homebrew System Transfer going from [ New series → Old series ]. I rather not walk you through this route due to concerns your n2DSXL and o3DSXL could end up getting banned by Nintendo the moment the o3DSXL signs on and presents itself with the n2DSXL's
Friend List. The Miis you should be able to get away backing up and restoring, but the
Friend List I'm not entirely sure this gets check against Nintendo's server side records.