Change an individual unit's attribute (armored, massive, biological) mid game, like with a data editor ability or trigger.
I would think if you make a morph ability and it changes a unit to another unit this is possible. I haven't tried it but making a dummy ability that just changed the unit into a new unit with same actors should work. Just change the attributes on the two units. I am thinking of "lurker" versus "lurker (burrowed)" units.