Mechanicum vs. Mechanicus

[Captain General]

I know that Mechanicus is 30k and Machanicum is 40k, but, not being familiar with the fluff, why the name change?

 

CG

[CommodusXIII]

After the Heresy, the Mechanicum was integrated into the Imperium's new organizational structure as the Adeptus Mechanicus. Same with the Legio Custodes (Adeptus Custodes), the Legiones Astartes (Adeptus Astartes) and many, many other organizations.

[Carlson793]

It could also have been a shift from neuter (-um) to masculine (-us) nouns.

[Iron Hands Fanatic]

Also, the 30k Mechanicum is less a unified organisation, and more a loose alliance between independent forge worlds, with varying degrees of allegiance and power all tied through Mars, in a pseudo-feudal system. Some were closely tied to the Martian Mechanicum, some merely paid lip service and were effectively independent. The Forge Worlds were so factitious and powerful that before the Heresy there were fears that the Mechanicum might renege on their treaties and engage in a civil war with the Imperium proper.

 

After a whole chunk of the Mechanicum turned traitor or went entirely renegade during the Heresy, the Adeptus Mechanicus was formed to create a closer union between the Forge Worlds ostensibly to prevent the same thing from occurring again.

[Tyriks]

Also the Mechanicum was essentially a junior ally of the Imperium more than a fully subordinate member, working alongside them because of the Emperor's treaty with Mars. During the Heresy, most of Mars went for Horus so the planet had to be conquered anew and lost a lot of the freedom and autonomy they had.