swap internal sd card to external storage
Unless it's important to you that the storage volumes are swapped, in Android Marshmallow the sd-card storage can be added to your internal storage with Adaptable storage.

Adaptable storage "enables a user to set up his SD card as internal storage, and store applications, their data, and media on an adopted SD card." –Source

Note: A high-speed card (UHS-1) is highly recommended, since you'll be experiencing lag and slow application speed with slower/older cards.

To use the feature: 


Turn off your phone
Insert the SD-card
Turn on your phone
Choose "use sd-card as Internal Storage" (When provided with the option - it should appear as a notification)


Android will then set up the card for internal use.
This will make Android show your sd-card as part of the internal storage and be used as such. The proces also encrypts the card, so your data is safe.

Note: When the card is set up as internal storage, you won't be able to use the card as a normal sd-card, fx. plugging it out and into your computer to copy files won't work. To use the sd-card like this again at a later point, you'll have to reformat the card.