So I've recently tried out marshmallow and kind of like the adopted storage idea as I never find myself removing the micro sdcard.
I'm curious though if there is a smooth way to upgrade to a bigger or better sdcard later on. Right now I would probably attempt to rsync with adb to a PC and insert a new card, format it and rsync back. I'm not very confident this will work however.
Perhaps it's faster to mount both cards in linux using this method: How to decrypt Adopted Storage? and sync them there.