Look, he's my supervillain, my Cobra Commander, if the upgrade can be done within the mechanics legally, then he will get it... If it requires a factory then it gets done in a factory long before the players get involved. He's not just a raider, there is typically diabolical politics involved, in true Battletech fashion...
Maybe it was a factory made upgrade to test the effects of upgrading the whole line and it was "diverted" into his hands by his backers? Or any of a hundred equally convenient cliches I can use. >:D