Even in the old rules (German and English) never was stated that you couldn't do so (or did I miss something?):
Old 1st ed. Khronos: A player uses Construction Cards to upgrade a Religious or Military building that they control or any Civil building on the Age of Might of Faith game boards.
The number of cards necessary for the expansion is the difference in the cost of the original building and the newer version.
Edit: The example in the 1st ed. rulebook under "rule of hierarchy" makes it clear in my opinion:
The biggest Military building in this domain is the Red player’s Keep. It is possible to add a Watch Tower to this domain or a Castle, but not another Keep...
In the new German rules it is expressively said that upgrading is possible fom the smallest to the largest building size in one step.
Keep in mind that it is also possible to build larger buildings right from the start without building their smaller versions first (always assuming you have enough cards).