Well, the simple reason is that anarchy already happened.
And at the time you chose not to change. Having been in Anarchy at some point in the past doesn't give you carte blanche to change at any time in the future.
When 80% of the realm secedes, you're down to three nobles, and you're having periodic starvation, it's anarchy in all but name.
Names are important (if they weren't, then you wouldn't be here trying to get it switched), and game mechanics trump RP. Since your government state isn't Anarchy at this moment, you don't get to freely swap to a new style. So far, I personally see no compelling reason to change it, simply because it would be more convenient for you.
I don't see why it should be necessary for me to step down. We've already done the anarchy thing. Plus, if anarchy does happen, the peasants will get pissy, and the last region may revolt, so I can't really step down. But theocracy won't piss of the peasants, who are majority Sanguis Astroism anyways. It's just anarchy that would mess up the peasants.
Plus, my prestige is low-ish after lots of tortures. If I step down, I could be close to the range of being ineligible: which makes no sense, because proclaiming a theocracy should increase Hireshmont's prestige, not decrease it, as it increases his personal sway and power.
Sorry, but I'm still not seeing a compelling reason to do it. All I see is that doing it would be personally inconvenient for you to do it with the completely valid method at your disposal.
My understanding of Tom's e-mail (I guess I misread) was that he was suggesting that a dev do it, not so much that it's unclear if it should happen or not.
If Tom wanted a dev to do it without debate, he would have said so, or he would have told you to tell Tim to do it, or he would have forward your email to Tim with a "Do it, please" attached.
The game has a long-standing policy of NOT doing this kind of thing. In the more than 7 years that I've been playing, I don't think I've seen more than two name changes, and never a government style change. (Did someone maybe mention that it happened in Arcachon some time in the distant past?) You have the tools to do it yourself, in a completely IC way.
