Using the most recent refit rules from Strategic Operations, if you're changing a mech (i.e. customizing it), you can move around the armour slots all you want without any effect. You can download those rules from the main BT errata page. If you're just making a new variant of the main machine (i.e. you make up a new factory or field refit of a design, like you'd find in a TRO), you can also move the armour slots around any way you want.
There's no rules for just moving the slots around but making absolutely no other change, because there's no point in doing that.
