You can also order replacement nibs/front sections from
AZ and swap them out with the roller-ball parts.
Actually, the front grips (metal parts) are completely different between the two kits. I'm not aware of anywhere that has the metal grip available without purchasing the entire kit, so the only way it to swap from one kit to another. Just don't forget to swap the spring along with the other parts.
CSUSA used to offer some front sections complete with grip (brass with black enamel), feed and nib that worked as a swap. But they stopped selling those and now only offer the nib itself.
AZ offers the feed and nib for the baron, but not the metal grip that matches it.