How do I remove a Wheat Pasted poster from my wall without damaging the poster?

I recently moved into an old house and found some artwork pasted to a wall by a past tenant. The posters/prints are really well done and appear to be wheat pasted. I would love to hold on to these posters if I can figure out how to get them down without tearing or ripping them apart. Any suggestions?

The walls are made of plaster if that helps.