i don't know if this is valid for WP8.. but WP7 internal memory is always a microSD, this because of microsoft requirement for produce smartphone with WP7.
let's SUPPOSE that WP8 has microSD too for internal memory:
you must open your phone and replace 16gb microSD with a 32gb one. This takes 2 major problems:
1) hardware level problem: i don't think the micro sd will be in a "easy to remove" slot, you probably you will need to do some "soldering work" in a very little space, not so easy!
2) software level problem: even if you just copy the old 16gb content into a 32gb you can't be 100% this will work. Contens are totally crypted and MAYBE (i'm supposing all this) enlarge the main partition to 32gb needs to be decrypted when its 16gb large and then recrypted when its 32gb. I can't be 100% sure of this but it can happen. MAYBE the software will check even the hardware info of the micro sd and work only on certified ones, so this can be another problem
well, this is not easy, i doubt anyone will ever do this, but you can try to google it and hope in some result inside XDA community