Yes, they can, but you’ll need to remove the manifolds, so a new gasket is advisable.
To get the old plugs out, a screwdriver or similar is hammered through them, then levered against the block to pull them out. Clean up the hole, then hammer in the new ones, flange outwards. DO NOT hit the centre of the plug but the flange only; hitting the centre causes them to ‘dish’, so reducing their outside diameter and making them a loose fit. You do not need to apply sealant.
One extra tip to help fitting core plugs a little easier. Put them in your deep freezer for a couple of hours first, this contracts them and they fit easier and expand as they get back to normal temperature.