I accomplished this with a lot of distress.
Things I tried that didn't work:
-updating from 6.0.0 to 6.1.0. Made no difference. All my Switches were on 6.0.0, so I don't know about other firmwares.
-DevMenu account stuff. It just froze.
-copying 8000000000000010 file over with Goldleaf. Crashed instead of copying. Tried this before and after a Factory Reset (Initialization).
-replacing 8000000000000010 with hacdiskmount without doing a Factory Reset or with a Maintenance Mode reset that keeps Users and Saves.
-backing up the system partition and using hekate to just restore that. Definitely doesn't work.
-I tried updating all my sd files and found out that SXOS 2.6/2.6.1 is incompatible for loading REINX (using SX Pro dongle).
Preparation
1. Update PC drivers to recognize the Switch.
>Download Zadig. Run the program. Plug the Switch in with USB C cable. Select Nintendo Switch in the first drop down box. Select libusbK (v3.0.7.0) in the box to the right of the green arrow. Then press the install button. Reset PC. Repeat the above, to make sure it installed correctly. (You don't need to reinstall if it installed correctly, but if you have to install again, reset the PC again.)
2. Get file 8000000000000010 from the files shared in this thread.
3. Get hacdiskmount. For USB NAND modificaltion: memloader, TegraRcmSmash. For NAND modification: hekate_ctcaer
Step 1: Make a NAND backup (I used hekate>tools>backup>full emmc backup). This step is ESSENTIAL, even if you are modifying your NAND by USB since any mistake can be undone without bricking. (I recommend backing up Boot 0/1 also. Sometimes they are necessary to restore the NAND.)
Step 2: Copy all of your saves with Checkpoint or Edizon and note anything special about your Users so you can recreate them, because they will be deleted.
Step 3: Factory Reset. Settings>Format Options>Initialize.
Step 4: Go through startup and create a simple user (it will be overwritten).
Step 5: Replace 8000000000000010
>Option 1: modify and restore NAND dump
>>Step 1: Run payload biskeydump.bin, press a volume key to create device.keys. Remove the SD card, insert in PC and open the file with Notepad.
>>Step 2: Open hacdisckmount with administrator privileges. Select the NAND dump. Select the SYSTEM partition then Saves folder. Delete 8000000000000010. Copy the replacement file into the folder. Unmount. (You need the biskey extracted earlier. Copy and paste the whole key in the first box. Note where it splits and copy the second half into the second box.)
>>Step 3: Restore the NAND backup. (With hekate, you need to move the nand backup to the root:/backup/?????/restore folder. "?????"=variable number)
>Option 2: modify the NAND by USB (this is faster since you don't need 30 min to restore the NAND)
>>Step 1: Copy contents of "sample" folder from memloader to SD root.
>>Step 2: Enter RCM with USB C cable connected to PC. Drag and drop memloader.bin onto TegraRcmSmash.exe. On the Switch use the volume/power keys to select ums_emmc.
>>Step 3: Open hacdisckmount with administrator privileges. Select the NAND dump. Select the SYSTEM partition then Saves folder. Delete 8000000000000010. Copy the replacement file into the folder. Unmount. Power off Switch (hold Power for 10s).
Step 6: Run the REINX payload at least once (SXOS/OFW won't work yet).
Finished: You should now have a linked profile. You can rename it and use it as you like
Follow up: To restore saves with Checkpoint, you have to run the game at least once before you can see the save and assign it to a new user. If restoring saves to multiple profiles, you may need to start the game with each of the new profiles once.
Note: You will lose all installed games and updates. At this point, I reformatted my microSD to Fat32, since exFAT can be corrupted by any homebrew app, and since USB installation of NSP is much faster and easier. I do keep a free 64 GB sd card on hand with payloads and homebrew files so that I can create a NAND backup or install a large XCI file.