Connect your smartphone to your PC.
Open command prompt and type:
adb shell
sm list-disks

The second command will look for the disks available for adoption. Note the disk ID in format 000:000. enable adoptable storage
Next, you have to decide if you want to use the microSD card as an internal storage in its entirety, or if you just a portion of the SD card to be treated as a normal SD card. Remember, saving some percentage of the card as the conventional SD card will help you store data like a normal card. You can plug out and plug it into your PC to get access to this data, including your photos or videos.
Once you have decided how much you want to commit to the internal storage, run the following command:

sm partition disk:000:000 private (000:000 will be replaced by your disk ID, and private will be used to commit 100% of the SD card as adoptable storage.)

OR

sm partition disk:000:000 mixed 50 (to split your SD card 50/50 between adoptable and regular storage. For 128GB value 44 is about 64GB)


The system will take a while to partition the disk. Once itłs done, you can view additional internal storage under Settings > Additional Settings > Storage. The apps will be automatically installed to the storage with the most space available, but you can always move things around.




Prerequisites:
Phone using Android 6.0
SD Card that will get erased
ADB access (developper menu + usb adb enabled + adb authorized)

Commands:
macbook:~ jean$ adb shell
shell@p1:/ $ sm list-disks
disk:179,64
shell@p1:/ $ sm list-volumes all
private mounted null
public:179,65 mounted 9F29-15FD
emulated mounted null

At this point I have a SD formated from the phone. Old cards are public and SD card device id is disk:179,64.

shell@p1:/ $ sm set-force-adoptable true

set-force-adoptable should not be needed. But I'm lazy and I won't do it again to confirm, so there it is.

shell@p1:/ $ sm partition disk:179,64 private

That line repartition and format, it takes time.
shell@p1:/ $ sm set-force-adoptable false
shell@p1:/ $ sm list-volumes all
private:179,66 mounted 0ef3b40b-103f-4331-b0ce-92f105f8xxxx
private mounted null
emulated mounted null
emulated:179,66 unmounted null

This is a win, the card is now mounted as private (internal).
Now, in Settings->Applications, you will be able to move even those apps that didn't want to go to external storage.