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.
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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.