NO, you cannot change the shape afterwards, but you can choose any design for your steel defender. After you have one, you can easily create a brand new one after a long pause. Again, you can choose a new design for your steel defender by giving him two or four legs that don't change any aspect of the steel defender.
ERLW PG. 61
You determine the appearance of the creature and
whether it has two or four legs; Your choice has no effect
on his game statistics.
At the end of a long break, you can create a new steel
Defender when you have your blacksmith's tools with you. If you
Already have a steel defender from this position, the first one
you die immediately.
Personally, I like the monkey because of its humanoid fingers and limbs. I think a steel defender monkey could anatomically wear gauntlets with ogre power and turn it into a gripping machine!