Sealing elements for well packers and bridgeplugs may be constructed with magnetorheological elastomers or foams disposed about electromagnet windings set in an internal mandrel. The elastomer or foam stiffness properties are formulated for a low setting force compliance in the absence of a magnetic field. After deployment, the tool winding is energized to engage the magnetic field and raise the stiffness properties of the elastomer or foam. When the magnetic field is activated, the packer or bridgeplug has greater structural resistance to unwanted pressure displacement. When the field is de-energized, the sealing element relaxes to permit resetting or removal.