It didn't actually turn into anything, its parts were just interchangable, so you could disassemble and reassemble it in different ways.
I think it was this:
Biotron : Biotron is the workhorse of the Micronauts, and is probably the biggest (at least in weight and bulk). He has a motorized track on his back for motion while lying down, and he also walks by himself, albeit rather slowly. There is a clear plastic cockpit on his front where a figure can be placed to pilot him. Biotron's hands are spring loaded clips and his arm and head sockets are motorized. Some of the MEGO literature suggests that Biotron is actually the leader of the Micronauts, or at least their greatest weapon against the evil Baron Karza.
http://www.micro-outpost.com/pictures/giants/giants.htmlOh okay.