Trying to upgrade a 950 flame sub alloy plate using a 951 arc sub alloy plate (950 better stats) and requires an upgrade module. Looking at a prodigal helmet, 950 to 951 upgrade just takes 1000 glimmer.
It's because the elemental affinity is different. So it requires the module. If the plates had the exact same model style and affinity it should only require glimmer. I'm not a fan if this infusion style. I hope it changes in the future.