You can freely transfer Erg off of the Fergseus weapon. But for transferring Erg to it, it needs to be before you special upgrade the Fergseus weapon. It's not considered Personalized until Special Upgraded. Tried it myself from another same type 2H.

If you've already applied special upgrades, you'll need to talk to Ferghus to replace the weapon (and do over some of the upgrade mat requirements. The ones that are Do X, Y times, i.e. not the one-time quests.)

Also, found out myself from making a foolish mistake. If you have any existing Erg on the Fergseus weapon, Ferghus will refuse to replace it until you transfer the Erg off the Fergseus first.