When you leave a tier and enter one further up the old machines become obsolete, how about an alternative recipe for next tier machines that include the full lower tier machine in the place of the LV/HV/IV/EV machine hull.. So for example the HV Alloy Smelter could have the HV Machine Hull in the centre or the MV (prev tier) Alloy Smelter.

Daft idea after thinking about it as it would bypass the reqs for stepping up.