The bad news: Using this expanded storage will corrupt your files.
After the update, if you insert an SD Card that the phone doesn't recognize as usable (ex. a wiped card, one used as 'phone storage' for another device, or one formatted as EXT4 or NTFS), you will get a notification. Tap that notification and you will be sent to a settings screen with a Format this SD card dialog already popped up onto it. Tap CANCEL on the dialog and you will be able to select Phone storage as shown here: https://www.makeuseof.com/tag/move-a...-apps-sd-card/ (Just tap CANCEL between steps 1 and 2).
After the procedure, it will seem like everything went swimmingly. However, if you had a lot of files on the internal SD Card and chose Move content, many of the images/apps/data that had been building up on the device are now corrupted. Images don't load, apps crash, and so on.
I have reproduced this on two different 6.1's and two different brands of SD Card. The files appear to get corrupted at random 4096-byte boundaries.
If you are a new phone purchaser or one who hasn't yet used an SD Card with your device, configuring expanded storage (which is what many 6.1 owners will want) will corrupt the apps, images, and other data that gets moved to the adopted storage.
I sure hope Nokia fixes this soon. It will be a nightmare for some users.