If the glue has not become brittle and the plants are not falling off or broken, I would not try to remove the plants and remount them. I would also not try to perserve them with spray or even enclose them in plastic. They have remained in great condition without them if I understand you right.
You can wrap each sheet in that thin acid free archival paper or place a sheet of acid free paper on the top and bottom of each mount. Keep out of the light, dry and away from bugs-it should be fine for many more years. If you can, buy an archival box of the same size to pack them in to keep them flat and intact.