Depeds on the adhesive holding it on. If a common wallpaer paste was used, it's a water-soluble wheat (starch) glue.. You can try soaking, but depending on the thickness of the cork perhaps not possible. Most expeditious might be to just not try to protect the plaster, just go after the cork with sturdy scrapers, then spend a bit of time with spatula and spackle to smooth over any scratches. Much faster in the long run. Less messy, too, since your debris is dry.