1. Move all the apps, photos, etc., over to the phone. If there is to much, try moving photos, music, etc (not apps) over to a computer (via USB cord) and then move the apps back to the phone's memory.
2. Then reset the current card as "External" storage again and remove the emptied card from the phone.
3. Insert the new card into the phone and once mounted, use the instructions included below to change the card to be "Internal" storage again.
4. Once it has been made "Internal" storage, you can then move the apps over and copy back the data you had saved to the computer (if some was).
5. NOTE: Before going through all this, check your phone's specs to make sure you know the limitation of what size card it can take. All phones have a max storage size card they can handle. Many will accept sizes above what the specs say, however then you run into complications like data corruption, etc.
First, you need to be using a phone that is running at least Android "Marshmallow" (or newer) or be able to upgrade to it. Second, you will need the instructions I am including below. And third, you need a phone that has not been restricted by the manufacturer from making this change. Which does happen.
NOTE: If any of the steps are missing on your phone, or they don't work for you, then you have a phone that either has an older Android version or has been restricted by the manufacturer.
How to turn the "External" SD card into "Internal" storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.
How to turn the "External" SD card into "Internal" storage on "Nougat".
IMPORTANT NOTE: Be sure you start with a blank SD card. When the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device), so anything on it prior would be gone.
1. Go to "Settings", and then select "Storage & USB".
2. At the bottom of the list you should see the SD card's details, including the option to format it and make it "Internal" storage.
3. Once this is done, reboot the device and you can start running things from the card.
If you don't reboot the phone, some things may not work correctly, so make sure you do the reboot.
Once a micro SD card is set as internal, it becomes encoded so you cnnot use it on any other device. Unless you can upload the data from the card to the cloud, a computer or other storage outside the phone and transfer it to the new card, you are screwed. And if you can transfer everything you need to the new card, I would highly advise setting it up as PORTABLE storage. Because, if the phone you're using breaks or you want to upgrade to a new phone, portable storage is a lot more sensible for future-proofing your valuable media, files---especially valuable personal files, financial or legal, you might need in the event of an emergency, family or otherwise. Contrary to all the advice on converting to internal, I think it's the worst you can do. BTW, If you use Nougat or later, you can move the data storage portion of non-widget apps (like the ones resembling programs on your computer) to the SD card using the onboard apps list.