You got 20mg

of memantine or 20mg in general? In a medical paper they used 60mg memantine which os what I'd recommend as a starting dose as long as you don't mind some dissociation.
May I ask where you got the memantine from, doctor of internet?
Yeah, one gradually replaces the opioid with memantine.
Memantine and pregabalin shouldn't bite each other but still it's better not to use too many substances. I first used the pregabalin to taper as much as possible and then both drugs to jump off the morphine and it worked well. Just be aware that pregabalin can be addictive on its own do maybe best to only use it as needed.
Good luck and please keep us posted, I'm always interested in reading about this method, and guess I'm not the only one. Maybe somebody else can give you more concrete tips about dosing. What's your morphine