As you have mentioned in your title, you are attempting to upgrade Legacy items? These are the "old" items we had prior to the SM Reloaded?
Yeah, these cannot be upgraded/boosted. They can only be used as fusion fuel and nothing more.
To get stronger in the game, you need to build a new mech. Most of the new parts have "upgrade potential" as indicated by the colored dots when you view their information. So, a common weapon may be upgraded up to mythical if it is available. Most leg parts can be upgraded from rare to mythical.
So, what now?
Legacy items - even maxed ones are no match against the current weapons...
Your best bet it is use whatever "good stuff" you have from your old inventory and mix it up with whatever new stuff you get and build up from there.
Most modules, shields, push weapons are still useful, so keep them.
And whatever you do... STAY AWAY FROM THE LEGACY CONVERTER without going over your inventory and equipping any usuable item you got.