For example i have two slightly different units and want to toggle the movement class or change the weapons.
So i want to transit from "unitxyz_a" to "unitxyz_b".
Hitpoints etc, should not be changed.

Is something like this already done somewhere?

change weapons:
give the unit both weapons, allow only one weapon to fire depending on which weapon is chosen. (via unit script)
easiest way to get some toggle button is onoffable=true

change movement:
max speed, turn, acceleration can be changed via simply calling a function:
http://springrts.com/wiki/SetMoveTypeDataExample

toggling the movedef of a unit is not possible though. (cant make a ship into an airplane)
Basically you can only change how a unit moves but not where it can go. See also meh

There is a "morph gadget" which basically turns one unit into another unit. In conflict terra and S44 it is used to "deploy" units, maybe that is what you want.

You will run into issues with unit targeting/behavior if the alt weapon has a shorter rang..