If you don't then they are unusable. Unfortunately I deleted quite a few old weapons thinking they were just underpowered in EoF. Then it dawned on me to try infusing one and testing it in the range. It was night and day. The problem is to infuse a weapon from 10 to 520ish takes 24,000 unstable cores which is ridiculous. I played for hours yesterday and was only able to upgrade 4 weapons.